Customs at Elstree:
Go to the tower for a customs form ('general aviation report') and give it to them completed. They will sort it out then. Apparently they charge you for this service even if you print it off the internet and fax it yourself (not typical Elstree or anything, that...
) so you may as well.
If you're coming back at a very different time from planned (especially earlier) get in touch - could cause problems.