Roy,
Great project of a rare version of the Sexton, so not a lot of info known. But let's start digging!
There is another survivor in Pakistan of all places - see the thread
Sexton Gun Position Officer. The full list of survivors can be found here:
http://the.shadock.free.fr/Surviving_Sextons.pdf
Is yours the one which was at Pounds Yard in Portmouth until the mid-1980s?
Regards,
Hanno