One of the things I most enjoy about going to the theatre is a really good opening night, and The Audience at the Gielgud last night was just that: a fascinating play, a brilliant cast, imaginative production and an appreciative, star-studded audience, with Sheridan Smith sipping champagne to my left, Neil Kinnock glowing pinkly to my right and Ben Brantley of the New York Times twitching sagely on the aisle.

And yet, as usual these days, half the critics seemed to have slipped into previews, the soft option for dead-line merchants whose print editors are increasingly reluctant to change their news pages late at night.

They missed not only the true moment of revelation, the official unveiling of, in this case, a royal monument, but also such cherishable minor mini-dramas as the last-minute arrival in the centre stalls of Nick Allott's partner, Christa D'Souza, at each curtain-up, and the distraught figure of A-List snapper Alan Davidson on the pavement outside, forbidden entry for some reason at the curtain calls.

I was minding my own business on my way back to the stalls in the interval when I heard David Miliband ask out loud what the Gielgud Theatre was known as before it was the Gielgud, so of course I told him. He then wanted to know when the name had been changed and when Gielgud had died, and I obliged on both counts, throwing in a couple of Gielgud anecdotes gratis.

This emboldened me to ask the affable former Foreign Secretary and "lost" Labour Party leader, who doesn't know me from Adam, whether or not he felt the representation of the Queen's weekly audiences with her Prime Minister bore any resemblance to the truth. He crinkled his charming little nose and emitted what I can only describe as a negative guffaw.

Of course he no more knows what does go on in these meetings than do any of us. They are off the record and private conversations. But it is the peculiar brilliance of Peter Morgan's play - and Helen Mirren's performance - to supply some imaginative and thoroughly engaging answers that reflect on the role of the monarchy, the philosophy of public service and the relationship of royalty and politicians.

In this respect, the coruscating encounter between Haydn Gwynne's vituperative Mrs Thatcher and the patiently and momentarily cowed Queen is something akin to one of those feisty face-offs between kings and rebels, or usurpers, in Shakespeare's history plays. Mirren's monarch never quite claims the Shakespearean, medieval divine right of kings, but she staunchly defends her sense of duty as something she can only obey, not tinker with, and that's undoubtedly something we all recognise as true.

At the same time, Mrs Thatcher seeems to be justified in her anger at being labelled "uncaring" in a Palace leak to the Sunday Times over her foreign policy and refusal to apply sanctions to South Africa; in which country, the Queen neatly points out, her "explorer" son Mark (referring to his wayward sense of direction in the desert) has business interests. Crumbs. And again, ouch.

This scene crackles with danger as it edges towards direct political controversy, something we know the Queen avoids at all costs and at all times, on or off the record. But that's the imaginative prerogative of drama, although Sunday Telegraph critic Tim Walker, in his Daily Telegraph gossip column, has predictably attacked the play for pillorying Baroness Thatcher - it unravels, briefly, a racist remark of her husband, Denis - and then lines up Norman "the pole-cat" Tebbitt to take a ludicrous pot-shot at "contemptible, vicious and anti-social" people on the Left.

The attribution of fallibility and human flaws in politicians is even-handed throughout the play, and the one thing it's definitely not is a piece of Leftist propaganda. I suspect that the Queen's blankness and serenity in public life is convincing precisely because that is how she really is; Morgan and Mirren pay due deference to her seriousness, acuity and diligence (in reading every single piece of paper in her red boxes, for instance) and then go beyond that to suggest a sad and lonely figure who responds most deeply to the least pompous and most "ordinary" of her Prime Ministers, John Major and Harold Wilson.

I wonder if any of the royal family themselves will go and see the play. I suspect not. When Alan Bennett's A Question of Attribution was on at the National Theatre, with Prunella Scales playing the Queen, I was terrifyingly asked by Princess Margaret if I thought Mr Bennett had gone too far this time.

The occasion was a lunch for Dame Ninette de Valois, founder of the Royal Ballet, at the Observer, where I worked at the time. I assured the princess - who hadn't seen the play, and had no intention of doing so, and was taking drinks from a special flask prepared by her own attendant - that Mr Bennett, while sailing a little close to the wind, perhaps, had nonetheless sketched a protrait of the Queen that was both humorous and sympathetic. My editor, Donald Trelford, breathed an audible sigh of relief; I think he was half expecting me to say that, alas, Mr Bennett hadn't gone nearly far enough in exposing the monarchy as a canker on the body politic and public life of Britain.

She wasn't buying my defence of the play at all, smelling treachery and insolence in equal measure, and she made it quite clear that it was no business of a Royal National Theatre (as it was then more widely known) to take aim at her family, however tactfully.

How times change. This play would have been unthinkable fifteen or twenty years ago. Now anything that shows the monarchy in a good light - as this play most certainly does - is a bonus for them.

The royal family have clawed their way back into public favour since the death of Princess Diana, despite Prince Andrew's oafishness and Prince Harry's antics, to such an extent that John Major's recommendation in the play that the Queen should de-commission the royal yacht Britannia as an economical gesture of goodwill now sounds over-harsh and unreasonable.