"There is no violence, no sex, no overt drama about it at all - it is just good talk. It is so unfashionable, it is almost revolutionary."

Michael Pennington, co-founder and formerly artistic director of the English Shakespeare Company, is enthusing about The Best Of Friends, in which he stars alongside Patricia Routledge and Roy Dotrice.

Based on the real-life friendship and fizzling correspondence between Dame Laurentia McLachlan, the Roman Catholic Abbess of Stambrook (Routledge), acclaimed playwright George Bernard Shaw (Dotrice) and the man who introduced them, Sir Sydney Cockerell (Pennington), The Best Of Friends unfolds as they share gossip, ideas and experiences via letter against a backdrop of two world wars.

"It is essentially three people discussing love, life, their religious faith or lack of it," says Pennington. "Sydney Cockerell, apart from being director of the Fitzwilliam Museum in Cambridge, had a great talent for friendship. He took a real interest in people and had an enormous number of friends - including very famous ones, who trusted him with their private business a great deal.

"He was constantly introducing unlikely people such as Shaw and Dame Laurentia and wrote a hundred letters a day in a tiny spidery hand. He viewed friendship as the most precious thing in life but said, 'It is like a plant that withers if you don't tend it.' So he spent all his life tending very many plants."

The Best Of Friends recalls an era when friendships were fuelled by longhand letter writing, long before email and text messaging obliterated the requisite muscle.

"The language is wonderful," says Pennington. "These are people who really knew how to express themselves wittily and well."

While the three actors share the stage as if they are in the same room, it is clear they inhabit separate worlds. Shaw and Cockerell are in their respective studies and Dame Laurentia is enclosed in the nunnery where she spent 60 years.

The content of their letters is turned into bubbling conversation, as if they are talking to one other.

"Their characters are so interesting," says Pennington. "Dame Laurentia has a very worldly sense of humour. She can be quite rude, in fact.

"She, who was the enclosed nun, was quite a lively person with a very earthy sense of humour while, conversely, Cockerell, who was free as the wind, was quite uptight.

"Shaw, too, was a mass of contradictions. He was a great and brilliant man, on the one hand witty and generous and on the other, arrogant and autocratic."

Starts 7.45pm, 2.30pm mat Thurs and Sat. Tickets £15-£25. Call 08700 606650.