Thanks for the replies. Piecing it together out of CAP 393 as suggested, WHOPITY seems pretty spot on, though it seems the Interception Procedures and Certificate of Registration only seem to be necessary on an instructional flight involved in international navigation.
As I think has been mentioned though, what the rules are and what actually happens are probably two different things ! Sounds like the CAA have been aware of the impracticality of this in the past too.