In Tom Stoppard's last stage play, Rock 'n' Roll at the Royal Court seven years ago, the Velvet Underground met the Velvet Revolution in a highly personal meditation on rock music and political upheaval in Prague; a second strand unravelled with a philosophy tutor's daughter haunted by the Pan-like ghost of Syd Barrett of Pink Floyd.

Now, in celebration of the 40th anniversary of Pink Floyd's tremendous album, Dark Side of the Moon - which sold over 50m copies and hung around the top of the charts for 15 years - Stoppard has written an hour-long accompaniment to the Floyd music involving moral philosophy, ecological meltdown, madness, thought waves and the limitations of human happiness.

Tom Stoppard (centre) with Rufus Sewell, Iwan Rheon, Amaka Okafor and Bill Nighy
Tom Stoppard (centre) with Rufus Sewell, Iwan Rheon, Amaka Okafor and Bill Nighy

Broadcast last night on BBC Radio 2 - and you can hear it for seven more days on BBC Radio's iPlayer - Darkside was a wonderfully depressing jeu d'esprit, a sort of Wizard of Oz with word play and classy jokes ("It's a success for the green belt," cried a victorious politician, "and we're going to build on it") in which a philosophy student befriends a boy who has been hit by a runaway train and, in the company of her tutor, an Ethics Man, seeks the answer to various questions.

Such as? Well, what is the secret of life, perhaps; or, do you believe in jugglers you hear on the radio; or, what is the good in the world if kindness is only a form of selfishness and there is no such thing as altruism? You can put all that in your pipe and smoke it, Stoppard seems to be saying, and try not coughing to death.

The dialogue was lightly laced through the music and, this being radio (and Stoppard's a past master, literally, as he started writing for radio in the first place), the action could be compressed and large incidents confined to a phrase. The ice was melting, we were dying of consumption, the weather forecast was a state secret... and Emily, the student, was liberated on the dark side of the moon with all the other lunatics on the grass.

Emily was delightfully spoken by Amaka Okafor, whom I last saw play a very creditable Miranda when she was in the Unicorn Theatre company for a couple of years. And what a cast chipped in around her: Iwan Rheon (Moritz in the British premiere of the musical version of Spring Awakening) was the boy, Rufus Sewell (Stoppard's alter ego, an intellectual bouncing Czech, in Rock'n'Roll) was the tutor, Adrian Scarborough a stranded Fat Man and Bill Nighy the mysterious wizard, or witch doctor, Mr Antrobus.

I miss the precision and meticulousness of Stoppard in most other plays, which is why I came over a little unnecessary at the spanking new Park Theatre on Friday night. Clive Francis has tinkered a bit with Ben Travers' 1927 farce, Thark, and hit a few bum notes. When the two men become pyjama pals in the haunted house, there's a certain amount of joshing and an exasperated cry of, "You are awful in bed." So far so Eric and Ernie. But Francis has Sir Hector Benbow (played by himself) whip out a hip flask and offer it to James Dutton's gormless Ronny, his nephew, with the insertion, "Would you fancy a quickie just to rock you off?"

Sorry, no can do with that. Wrong sort of laugh. Travers was a careful, if not a brilliant, writer, and he deserves respect. I don't know what Stoppard thinks about him, but I know what he thinks about people who really can write, even if they are critics (Kenneth Tynan and Clive James for starters); he himself could never remain a critic (he started as one on a Bristol newspaper) because, he once said, he never had the moral courage to pan a friend; or, rather, he had the moral courage never to pan a friend.

He also once said that he would just as readily read Edmund Wilson on Sophocles as read Sophocles himself, an aphorism echoed by David Hare in The Observer on Sunday in his appreciation of the retiring film critic Philip French:

"Faced with a choice between watching Jonathan Trott bat and reading Michael Atherton writing about Trott's batting, I would pick Atherton every time. It may be a back-handed compliment to say that French has spared me the misery of sitting through thousands of terrible films, but he has often made reading about films more enjoyable than seeing them."

Mind you, the analogy broke down later in the day as Jonathan Trott led a magnificent late surge to victory in the final Ashes Test at the Oval. It was high excitement all the way for as long Trott and Kevin Pietersen were batting. But with four overs left and only 21 runs to get, the umpires whipped out their light meters - the floodlights were on, for heaven's sake! - and ushered the players from the field. That was one hell of a dark side of the setting sun.