The question is - did you "act as a member of the flight crew" on that flight.
(
http://www.caa.co.uk/docs/33/CAP393.pdf article 79).
Article 50 says you need a licence to act as a member of the flight crew.
I'm sure somewhere else there is a "you can do anything you need to in an emergency to preserve life" or similar clause but I don't have it to hand.
I'm sure that once you say the magic words "mayday mayday mayday" then you can legally be a member of the flight crew, so everything would be legal
and you can then log it - does someone have the reference?