Just gone and checked it out seeing as it's been standing still since this morning and Gambit might be right ...
There's little oil left in the system and it appears to have all been punted in to the coolant system. There no obvious contamination, and I would not have expected the oil level to decrease if the gasket had gone between and oil / coolant gallery.
I'll try and get a compression test done too, but she's still running perfectly on all 4 cylinders, plus if the seal has gone between the oil and cololant should the cylinder compression be ok?
Might be a good time to move this to the garage section mods ??
Are oil coolers something that do 'go' then? I've never heard this mentioned before that's all ..