Chief among the many pleasures of Alan Bennett's double-bill Single Spies is that it gives audiences the chance to see an actor - in this case the excellent Diana Quick - represent the profane and sacred in the same evening. First comes the notoriously foul-mouthed thesp Coral Browne in An Englishman Abroad, the tale of how she exported Savile Row tailoring to Moscow for the benefit of the exiled traitor Guy Burgess. Post-interval, actressy mink coat swapped for homely twin-set and pearls, she shows us Her Majesty Queen Elizabeth II in A Question of Attribution. In this we find the monarch, for once at a loose end, pottering amid the Buck House art collection, entertaining fears about the provenance of some of the pictures there, even as she (along with the nation's spycatchers) is coming to doubt the authenticity of the man charged with their safe keeping, Sir Anthony Blunt.

How well Quick impersonates Browne will be obvious to anyone who saw John Schlesinger's 1983 film of An Englishman Abroad, in which the Australian actress played herself. The representation of Her Maj is no less deliciously authentic - I loved her "awestridge egg", pointed out on a cabinet among other relics of decades of so many State Visits. Like Prunella Scales and Helen Mirren before her, Quick has our monarch to a T (though I doubt if Bennett is accurate in showing her to be quite so fascinated by the tedious technicalities of the swimming pools, factories and the like which she is obliged to declare open).

Quick's co-star Nigel Havers also provides a relishable study in opposites as Burgess and Blunt. While it is true that both were upper-class, gay and treacherous, in other respects they could hardly have been more different - the first so rackety, so boozy and indifferent to anyone's opinion of him; the second so patrician, so proper and jealous of his position in court and academic life.

If the plays have a fault it is that neither dwells even momentarily on the human cost - the British agents exposed and executed - as a consequence of the pair's activities. Only in the approach of the down-to-earth interrogator (John Arthur), sent to urge Blunt to 'name names', is the audience reminded of the real and dangerous world in which their dirty trade was plied.

Single Spies can be seen at Milton Keynes until tomorrow. Box office: 0870 060 6652 (www.miltonkeynestheatre.com)