Albie sent this link (cheers). It’s the Pink Floyd gig at the Guildhall when they played the whole of Dark Side of the Moon for the first time ever anywhere (January 1972). I reckon someone sneaked in with one of those old cassette recorders and loads of batteries so the sound ain’t perfect by any means, but there it is – a bit of Pompey history.
PS Mick has sent me the link to the Wikipedia account where it says that it was premiered in Brighton but that’s not correct (there are a few books that have the story). It should have been Brighton the night before Pompey, but their gear didn’t work properly and they only played some bits. They had similar technical issues at the press launch.