Hi all,
A friend of mine is a lawyer, so I asked her opinion... she didn't really want to give one, but offered to ask a friend of hers who is an aviation lawyer. I got the answer this morning.
"There is no definitive answer to this but, she feels that asking for cost-sharing passengers by email should be ok, providing you know the people concerned and it's a private email... i.e. not posted to a publicly accessible forum or sent through a list-server. Obviously you need to make sure you're not breaching any of the other requirements like number of passengers, proportion of cost borne by each passenger, etc.
The CAA do not seem to have defined exactly what constitutes an advertisement, therefore they could be able to argue that almost anything is an advertisement, including word of mouth. In general you should be ok.
If you stick to the above and the CAA challenge you it should be straightforward to defend yourself."
Ivan.