Got the bleep ...

[gazza82]

and anything over about 7mph and get that really annoying constant bleep.

Anyway eventually traced it to the connection plug on passenger side. I'd tried to fit an led third brake light and in doing si pulled the cable a bit. In doing so pulled the hinge microswitch plug from its socket. Removed the plastic shield with three torq screws and could see it immediately. Now I find that the plug will not clip in so I've taped it up to keep it in place.

Still can't get third brake light working and suspect a broken wire as the old one doesn't either. My money is on the bend in the cables down by the boot hinge as they are where the loom travels up to the boot lid electrics. PITA to get to!!!