This is an update on my June 2003 comments which I had I posted based on my recollections of the videotape that I'd hired in the early 80's - but which I hadn't seen anywhere since.
So I was surprised to see the DVD listed for release in Australia in October (2004) and immediately ordered a copy. The text on the back of the DVD continues to mislead the viewer by referring to the film as "a high camp farce" and "cult British spoof". However it's clear after finding out more from the internet about McGonagall that the film is all based on the poet's autobiography - most of the circumstances are the same, they're just presented in a slightly absurd manner (but not much more absurd than the poetry itself). The poetry in the film is definitely the original McGonagall.
It's still a very funny film, and even before I found out that McGonagall was a real poet I appreciated the tragedy and pathos of the character. I think it was Spike Milligan who said that someone, or something, suffers in every joke that's ever been funny.
The DVD quality is very good considering the low price - so I'm hoping that Spike Milligan's "The Bed Sitting Room" can be made available too. Spike Milligan and William McGonagall had a lot in common in regard to their ability to make people laugh - but that was exactly the reaction Spike wanted, whereas William struggled to be taken seriously as a dramatic performance poet.