London Assurance, NT Olivier, London<br/>Love Never Dies, Adelphi, London<br/>King Lear, Courtyard, Stratford

An early Victorian comedy is a star-studded treat, but Lloyd Webber's plot rejigging enflames some 'phans'

Simon Russell Beale is a ludicrous bauble of vanity in London Assurance. This is the little-known comedy by Irish-born playwright Dion Boucicault, penned in 1841 and now revived with joyous brio by Nicholas Hytner.

A superannuated City dandy with his sights set on a rich young bride, Beale's Sir Harcourt Courtly looks like Lord Byron crossed with Mr Toad. A triple-chinned epicurean – vast paunch bobbing above absurdly balletic turned-out toes – he would have delighted the caricaturist Gillray.

As he totters off to Squire Harkaway's rural manor to woo Michelle Terry's aghast Grace, what Courtly doesn't realise is that his own son Charles (from his first marriage) is a rival already in situ. The latter is gallivanting under an assumed name, together with a rascally impostor called Dazzle – forerunners of the wags in Oscar Wilde.

Courtly is equally unprepared for the gusto of his fellow house guest, Lady Gay Spanker. Fiona Shaw is horsiness incarnate as the bounding aristo, and as she thwacks her skirts and hoots with merriment, he is smitten.

Hytner's production points up that it's the gentlemen who are the fools and sissies, most hilariously when Harcourt believes Spanker is ready to elope with him in disguise, and Beale rams a stove-pipe hat plus long beard over Shaw in her hooped frock, so she looks like a cross-dressed Abraham Lincoln, smirking as Beale scuttles around in a bonnet.

Some elements are less inspired. Matt Cross's brash Dazzle is, in fact, rather dull. But Nick Sampson is superbly droll as the unflappable valet Cool, and Richard Briers adorable as Spanker's gaga husband.

Punters unfamiliar with the play might feel cheated if they knew how many of the funniest lines aren't bona fide Boucicault. Who's complaining, though, when the cast's ad libs and other revisions (credited to Richard Bean) bring the house down?

London Assurance is a resounding hit, but can the same be said for Andrew Lloyd Webber's new mega-musical Love Never Dies? On the night I attended, there was an ill-boding announcement on the public address system. The Phantom, we were warned, had a sore throat.

So was our man in the mask going to wheeze his last before this fanfared sequel to The Phantom of the Opera even had a chance to run and run? Hell, no. You'd hardly have guessed Ramin Karimloo was poorly as he launched into the Phantom's lovelorn aria "'Til I hear you sing", with lashings of vibrato and a surging crescendo.

I guess coping with a virus is a mere bagatelle when you're a gothic horror who never gives up. Half Karimloo's head, when he removes his mask, is hideously worm-eaten, but he's none the less contrived to become a funfair tycoon in the years since he quit his Paris Opera lair. Having lost his beloved Christine to the good guy, Raoul, the Phantom has become Coney Island's Wizard of Oz-cum-Dr Frankenstein. He presides over a spooky, spectacular citadel of whirling ferris wheels and macabre automatons. And he's hatched a plan to lure back Christine.

Some self-styled "phans", fixated on the original show, are outraged that Lloyd Webber has rejigged the love triangle. Personally, I don't give a monkey's. Why not take a new slant, with romantic hero Raoul morphing into tetchy husband (top-class actor Joseph Millson, who also has a fine singing voice).

While Raoul hits the bottle, Sierra Boggess's Christine – yearning for a second brush with the dark side – is drawn into eerily echoing duets with Karimloo. Undeniably, Lloyd Webber can write a haunting melody, and everyone's going to come away from this show with earworms – tunes that just won't go away. But Boggess's climactic solo "Love Never Dies" seems to have been lifted from Sir Andrew's own back catalogue, and the syrup quotient is drip-fed Creme Eggs.

Finally, the monarch's decline, from riches to rags, is subtly charted in David Farr's excellent new RSC King Lear, with Greg Hicks as a hawk-faced potentate, with straggly hair and an incipient stoop.

The period setting is not specific: a medieval/Edwardian/ modern-day militarised state where the chandeliers flicker in Lear's jerry-built fortress. That ingeniously foreshadows the storm scene, when the King's world collapses and he goes mad in hallucinatory flashes of lightning.

The sub-plot's opposed brothers, Edgar and Edmund, are weak links. But Kelly Hunter and Katy Stephens are complex as Goneril and Regan, terrified of their cursing father as they turn cruel. Kathryn Hunter's Fool is like a wizened little boy, a nervous simpleton who can't stop speaking the harsh truth, and Hicks himself, even while withering into a demented tramp, regresses into poignant, childlike pretending games.

In an unforgettable cliff-top scene, in a haze of sunlight, his Lear stands crowned with wild flowers. As if playing at Red Indians, he fires a stalk of corn from his imaginary bow, then suddenly whips it round to pierce his own breast. Fantastical, comical and heartbreaking.

'London Assurance' (020-7452 3000) to 2 Jun; 'Love Never Dies' (0844 412 4651) to 23 Oct; 'King Lear' (0844 800 1110) to 26 Aug

Next Week:

Kate Bassett reports back on The Gods Weep, a portrait of corporate greed by Dennis Kelly