I had found this actually which did help. Last night i found a guy is Australia selling a C's for an R32 and im sure i read somewhere whilst on my quest that the skyline and s15 ones are the same- assume as they're 6 speed? anyway he measured teh missing alloy O ring for me and the sizes were:
outter D: 50mm
inner D: 45mm
thickness: 15mm
sound about right?