Well, the power of Google has tracked it down. Just doing a search on "greek island" play homosexual volcano pulls a New York Times review of a more recent production of Michael Sherman's "A Madhouse in Goa" as the first hit. Investigating a bit further, I find to my surprise that the production I saw starred Rupert Graves and Vanessa Redgrave. I have seen fewer than half a dozen London shows, and am amazed that I had forgotten the star quality of the production. What I do remember is that it really wasn't a very good play, and at one point Larry Lamb (playing the writer in the second act) started visibly giggling at the end of one of his scenes.
Anyway, just thought you would like to know. The birthday link (which I had also forgotten) is that the second act of the play explicitly references the Chernobyl disaster, which of course happened on my 19th birthday.