Yip, that scene is when Colin Grigson (Rik Mayall) has temporarily left the band and Vim Fuego (Ade Edmondson) is playing bass, although he normally plays guitar (and piano, on which he writes an original song called Imogen that bears a striking resemblance to Lennon's Imagine).
In that still Den Dennis (Nigel Planer) is clearly playing the guitar, as he does throughout.