'In principle I'd be delighted to write something for BtP,' he wrote in an e-mail at the very end of 1999, 'but I have a huge backlog of writing needing my attention at the moment, so I think that the reality is that I shouldn't make any promises … Meanwhile, have a Happy New Millennium.' Sadly, however, we embark on the said Millennium without him.
The BBC page about his life is here, and it mentions his playing guitar with Pink Floyd, though not the Procol connections.
Adams's famous 'Restaurant at the end of the Universe' (see illustration) was inspired by the explosion of sound as Mick Grabham's guitar comes in before the final verse of Grand Hotel.
Various pages relating to Douglas may be found at 'Beyond the Pale':
Douglas introduces Procol Harum at the Barbican in 1996
Douglas introduces Procol Harum at Redhill in 1997
Douglas interviews Gary Brooker at the Barbican in 1999
Stephen 'Doc' Wallace records a special Holding On using Douglas's voice
Procol mentions in MJ Simpson's classic biography of Douglas
Gary Brooker at DNA's posthumous sixtieth birthday party ... setlist and band personnel here
Douglas Adams's Memorial Service took place on 17 September 2001 at St. Martin-in-the-Fields Church, Trafalgar Square, London.
His friend Gary Brooker was one of the select band of musicians who paid tribute, with Holding On, a special favourite of Douglas's. Dave Gilmour played Wish You Were Here.
A brief mp3 musical extract from the BBC's (intermittent) webcast may be heard here
The entire service (2 hours 36 minutes) is preserved online at rtsp://rmv8.bbc.net.uk/cult/hitchhikers/douglasadams.rm; the proceedings begin at about 52 minutes, GB plays with Margo Buchanan at about 1:34:00, and Holding On follows at about 1:42:00.