Many thanks guys, the head is off my WR at the mo, the gasket went between pots 3 & 4.
Luckily the history is good so I can see that the head was refurbished by BR Motorsport in 1992 and it another refurb about 8 years ago, which was only 3 or 4 thousand miles ago.
So will get some collets ordered and compare, will post some pics later on