Having miles of 2.5 T+E dated prior to 2005 can I use this provided it
is (a) buried in a sand lined trench of suitable depth below the drive,
(b) marked with that yellow warning tape and (c) in some sort of
protective tube?

You should be using SWA, I'm afraid. It just shows how Part P is encouraging
people to do a second best bodge job, rather than do the job properly.

Christian.