Thanks for all the info chaps!
Doesn't happen very often now though - the Northern summer maintenance has been done in Canada for almost 20 years now, and the aircraft only come over to Europe for specific projects.
That's what I had understood to be the way things worked, so was surprised to 'see' one in the UK these days. Perhaps one of the Duxford visitors this weekend would be kind enough to post a photo in situ ?