The original daytime Countdown show was hosted by Richard Whiteley and, after this death in June 2005 (aged 61). C4 made a 22-minute tribute show:

Mr. Countdown: A Tribute to Richard Whiteley

According to IMDB (and Google AI – which is probably reading IMDB) Jimmy featured, uncredited, in this show within Archive footage. I’ve speed watched the below and I can’t see him.

So, that’s “nope” from me.

Suzi Dent and Carol Vorderman are included and the show is below, for anyone that’s interested.

For anyone that wasn’t aware – Jimmy did feature in Dictionary Corner in 2004 – here.