This is from Andy Walmsley's Random Radio Jottings blog:

Jottings

"His last radio recording was made at the BBC's Paris Theatre Studios on 7 March 1988 when he was a guest of The Spinners – they were celebrating their 40th and final year in showbusiness. Kenneth's diary entry reads: "Met the Spinners in the narration booth & then rehearsed with Terry Walsh [guitarist]…oh! it was a delight to see him again. Alas! we have both grown old since the days of 'R.T.H.'. Eventually I went on and did the spot in an enclosed space-they screened off the studio saying it made a good atmosphere but the truth was that it was a sparse audience."

Whether or not it was Terry Walsh in the YouTube video (which I think must be from *An Audience with Kenneth Williams" recorded in 1983) isn't clear - the accompaniment sounds pretty similar to the recorded version but that's not necessarily conclusive, of course.

Stop Press: IMDB gives amongst the credits for An Audience with Kenneth Williams:

Paul Keogh musician: guitar.

Paul's list of credits on All Music includes an appearance on a 1972 album called Writer of Songs by a certain Mr Harvey Andrews...

LFF