Who was the comedian in the late-'70s/early '80s who had "spook" and
"slither" as his catchphrases?  He wore a prison suit decorated with
arrows and had a salubrious appearence.  He appeared many times on
British tv at this time usually in short slots on variety programmes. 
He might have done the Royal Variety Show on one occasion.  I am not
sure whether he was English or American.

Dear eltel,

The name of that comedian was the American Roy Jay. He is mentioned in
this brief article to the "Bob Monkhouse Show" which was broadcast by
BBC TV in the early eighties:

"Never-seen-again guests included (...) ROY JAY, that American bloke
who used to dress up in a Dickensian convict's uniform with arrows on
it and say "slither!" and "spook!" a lot, did an ad campaign for some
dire snack treat, then vanished forever."

Source:
TV Cream: Vintage TV Shows
http://tv.cream.org/arkb2.htm

The description of his appearance leaves no doubt that Roy Jay is the
comedian you have in mind. Also, I found him mentioned in several
Newsgroup threads, of which the following provided me his name:

"Remember a few years ago there was a crap comedian on TV: he always
wore a prisoner's suit, and had his eyes darkened.  His catchphrases
were 'SSSSsssslither', 'SPOOK!' and 'You'll all be doing it tomorrow'.
Can anyone remember his name????"
Source:
Declan Higgins, "Crap comedian: What was his name???" Online posting
(1994-07-21). <rec.arts.tv.uk> via Google Groups.
http://groups.google.de/groups?q=%22Crap+comedian:+What+was+his+name%3F%3F%3F%22&hl=de&lr=&ie=UTF-8&oe=UTF-8&newwindow=1&selm=CtAKKw.BwC%40bfsec.bt.co.uk&rnum=1&filter=0

Answer:

"It was definitely Roy Jay (...)"
Source:
Alan Hart, "Re: Crap comedian: What was his name???" Online posting
(1994-07-21). <rec.arts.tv.uk> via Google Groups.
http://groups.google.de/groups?q=%22Crap+comedian:+What+was+his+name%3F%3F%3F%22&hl=de&lr=&ie=UTF-8&oe=UTF-8&newwindow=1&selm=310bg5%24lvj%40lyra.csx.cam.ac.uk&rnum=5&filter=0

Search terms used:
spook slither prisoner comedian (in Google Groups)
http://groups.google.de/groups?hl=de&ie=UTF-8&oe=UTF-8&newwindow=1&q=spook+slither+prisoner+comedian&sa=N&tab=wg&meta=
"Roy Jay" slither
://www.google.de/search?q=%22Roy+Jay%22+slither&ie=UTF-8&oe=UTF-8&hl=de&meta=

Hope this answers your question!
Best regards,
Scriptor