My general impression has been that Swarb was a star or co-star in his projects, and did not tend to serve as anyone’s sideman.
An example of this would be his withdrawal from the 2009 Witchseason Weekend. His statement referenced his preference not to be an accompanist for a string of guest singers.
I think this is very much the crux of the matter. If you’re helming a project you need people that are going to turn up on time, take advice on board and subdue their ego long enough to make the whole thing turn out to the best of the leader’s vision. With the best will in the world, I don’t think anyone would suggest Swarb as the first person to fit that profile. (See, for example, RT playing lead guitar for Clive Gregson on the OGWT or Danny Thompson’s many, many contributions to other folks’ work or, for that matter, Aly Bain’s ensemble playing on the Celtic Connections series).
The late-period RT/Swarb BBC TV session has been previously referenced as possibly being a generous TV-related payday for an old comrade who had found himself on his uppers.