Repair completed, not entirely invisible but better than it was to start with.
I settled for this stuff (
http://uk.rs-online.com/web/p/repair...sives/0850962/) which we used to use at work, black in colour a metal loaded epoxy.
(I just happened to have a bit lying around
)
Amazingly the meter still works, it must have had a real whack.
Thanks to everyone who replied.
Paul.