Between 1999 and 2003 impressionist Alistair McGowan and Ronni Ancona had a TV show called Alistair McGowan’s Big Impression. Broadcast on BBC1 S1 Ep1 went out on April 26^th 2000,

As mentioned on this post (about the role of a warm-up comic and other shows that Jimmy worked on) the names (of the comedians) were not recorded and so information is scarce.

This 2002 BBC website page says:

Not only does Jimmy appear in numerous television roles he also regularly warms up for the Frank Skinner Show, Baddiel & Skinner Unplanned, Alistair McGowan, Al Murray’s Time Gentlemen Please and the Alan Partridge Show amongst others

As Jimmy started to appear on E4 from 2001 (here) it’s like that this warm-up job would have been around 2000, so S1.

Since those early days Jimmy has crossed televisual paths with Ronni on a number of occasions. For further details see here.