The repertoire of the OAE is creeping away from the 18th century and into the 20th with such unashamed eagerness, it wouldn't be at all surprising to see them throwing up an urtext edition of "Hit Me Baby One More Time" in a few seasons. Last night, we got 20th-century French impressionism, including a work that was premiered in 1933. Some might call this expansion into the last century bold. Others greedy.

It's one of the fundamental rules of concert-going that in any given season there will be one piece that trips you up. And that piece will always be by Berlioz. No matter what new alchemical concoctions Boulez, Lachenmann, Ferneyhough or Rihm will throw at you, someone will programme something by the 19th-century French composer - usually something with a perfectly benign-sounding title like King Lear Overture or Roméo et Juliette - that will in fact sound more modern, more outlandish, more baffling than anything written before or since.

Having blazed a trail through choral music, Simon Russell Beale now focuses his attentions on the symphony in this new four-part series. At last able to put aside the mind-games and chicanery of his role as Home Secretary William Towers in Spooks (RIP), Beale emerged as an engaging and enthusiastic host in this opening episode. He wore his erudition with an ironic twinkle as he toured the garrets and palaces of Europe on the trail of Haydn, Mozart and Beethoven.

A problem child in any number of ways, Mozart’s Die Entführung aus dem Serail doesn’t always get the professional attention it deserves, certainly not from London companies. The opera’s last outing at the Royal Opera House dates back almost a decade, and you’d have to look even further back to find it in English National Opera’s performing catalogue. If a director manages to get past the knotty Orientalist issues of staging then there are those of the dialogue, not to mention two of Mozart’s most taxing vocal roles in Belmonte and Konstanze. All of which places the focus firmly onto semi-staged performances such as last night’s superb effort by Bernard Labadie and the Orchestra of the Age of Enlightenment.

In the last year of his life he was, as a colleague noted when we learned of Charles Mackerras’s death, the wise old gamekeeper in the spring forest of Janáček's Cunning Little Vixen. No wonder Mackerras, we were told last night by his conductor nephew Alexander Briger, wanted that most ecstatic celebration of the natural order for his memorial, just as Janáček had had it played at his funeral. Was it trivialised by an encore number from Mackerras’s deliciously arranged Sullivan potpourri-ballet, Pineapple Poll? Not a jot, mate.

In 1860 Wagner sent a full score of his recently published Tristan und Isolde to Berlioz, inscribing it: “To the great and dear composer of Roméo et Juliette, from the grateful composer of Tristan und Isolde.” The bonds between these two works go far beyond emotion, as last night’s inspired piece of programming from Simon Rattle and the Orchestra of the Age of Enlightenment proved. A phalanx of nine double basses watching over the swollen orchestral forces of the OAE set the tone for an evening whose “authenticity” was anything but dusty.

Six years after their triumphant Rhinegold, the unexpected partnership of Rattle, the OAE and Wagner have returned to the Proms, this time taking on the even greater challenge of a period Tristan – albeit only Act Two. Spanning the lovers’ move from private uncertainty to fully declared (and unwittingly public) ardour, this act has a sense of intimate detachment from the rest of the opera. Framed at the beginning and end by the receding calls of the royal hunting horns and the sudden return of King Mark, the central section sees even the faithful Brangäne melt away, leaving the two lovers alone to reach musically towards the purest expression of Wagner’s “true happiness of love”.

It’s a section that makes sense as an excerpt, but demands much from singers, denied the aid either of sets or the dramatic build-up to such intensity. Leading the way was Violeta Urmana’s Isolde, whose tone is so perfectly produced as to bypass the forceful, big clichés of Wagner singing, even in a space like the Royal Albert Hall. With a purity that sits well with this determined young heroine, she captured both the expansive lines and single-minded focus of Isolde, coming into her expressive own in the charged opening discussion with Brangäne.

It was lovely to see baroque darling Sarah Connolly, more often heard performing the likes of Handel with the OAE, making the transition along with the orchestra into this wildly different repertoire. Her Brangäne was ripe with gorgeous tone colours, particularly the Watchsong, her position at the back of the Choir lending it a dramatic potency matched only during the evening by Franz-Josef Selig’s heartbreaking “Mir dies?” – surely the most welcome of musical interruptions. Gravely noble and surprisingly well projected, his King Mark was a late star of the evening, taking the pressure off the massively over-faced and struggling Ben Heppner.

I’ve never had the pleasure of hearing Heppner in good voice, and while I’m sure accounts of his vocal and dramatic capabilities are true, the fact remains that his notoriously troubled voice is letting him down more often than not these days. When he doesn’t actually pull out of a project he frequently sounds as though he should have, and last night was no exception. It never bodes well when a tenor is cracking and straining within the first 10 minutes of Tristan, and vocal (not to mention dramatic) suicide to attempt to carry on regardless.

Carry on Heppner did, however, with a gripped tone and adolescent hope-and-a-prayer approach to top notes that simply weren’t on offer. The whole experience, far from transcendent and pure emotion, was one that left the audience nervously anticipating the next crash. Credit must go to Urmana, who staunchly continued to emote despite the intonation issues next to her, and the fact that Heppner was giving her all the dramatic impetus of a tea cosy. Heaven help next year’s Covent Garden Peter Grimes.

Translucent and flexible, the “authentic” interpretation of Rattle and the OAE was so musically convincing as to overcome its novelty. The sharper than baroque, less sharp than contemporary concert pitch of A=437 took a little getting used to, but together with German instruments brought a darker timbre to proceedings that was particularly striking after the bright French textures of the Berlioz.

I can’t help feeling that with his love of technical innovation and excess Wagner would have welcomed the greater brute force of the contemporary orchestra, but the clearly defined textures of the period interpretation made sense of his multi-layered motivic writing, drawing the eye more frequently beneath the dense surface of massed strings. And you can’t argue with the bluntly atmospheric impact of five single F horns – an absolute joy.

With his prologue of Romeo et Juliette, Rattle placed the Wagner in context both musically and texturally. Providing a different period sound for this work (using instruments made to the French model familiar to Berlioz), the orchestra showcased the expressive range of an approach that goes beyond mere academic curiosity.

With contemporary-instrument Rattle still to come at this year’s Proms, the OAE have laid down a colourful, many-textured gauntlet for the Wagner of unlikeliest of competitors, the Berlin Philharmonic.

If Beethoven’s Third Symphony Eroica was the seismic upheaval, not just for Beethoven but for the entire symphonic movement, then the Second Symphony was most certainly the pre-shock. And we can be precise about the moment that Beethoven blows the Haydn model right out of the water and glimpses the far horizon of his brave new world: it’s the extended coda of the first movement where a devious harmonic shift sets collision course for the rip-roaring climax in which the trumpets turn wilful dissonance into exultancy.

Go on, admit it. You’ve done it too. Someone is talking in your vicinity and you’ve turned round to give them evils. It’s a manoeuvre I’ve been perfecting for years. The classic rebuke is in the speedy twist of the neck, a withering glance in the perpetrator’s general direction (but not, crucially, into their eyeballs: too confrontational) followed by the slow, affronted turn back to face the front. For one night only, the gesture says, you are singlehandedly ruining my life. I didn't pay more than I can afford to listen to you whisper through Beethoven/Shakespeare/”The Great Gig in the Sky” (yes, they can strike anywhere, any time).