This was discussed at length a couple of months ago, have a search for Charity Flight.
AFAIK So long as the pilot does not receive any money for the flight it does not constitute a commercial flight and the pilot must make his contribution to the flight. But I could be very very wrong so I suggest:
DO do a search for the other post, it went into it in some detail.
Edited to add link to the thread:
http://www.pprune.org/private-flying...n-flights.html