Thanks for the advice given so far by everyone. I will write to the CAA for an official response, the content of which I'll share when it becomes available.
What concerns me is that as far as I was aware a PPL could not have his flying paid for by someone else (a company or individual). i.e.: the pilot must make a contribution to the costs. This would seem to suggest (if I'm correct) that the pilot should not claim the entire cost of the journey, rather some proportion of it.
Presumably matters become even more complex when a work colleague is taken (or maybe it simply defaults to the "standard" cost sharing rule).
Fujiflyer