Thanks for the suggestions and help everyone. Oddly, the leak stopped this weekend - was at Blyton both days for the sprints and I put paper towels on the floor under the sump after each run and when it cooled down overnight and there was not a drip to be found. The leak is clearly very very slow. I probably need to change the oil anyway as its had the same stuff in it for a few events now, so I'll drain and remove the sump and take a look at it (welder flaw spray is next in my google list!). One question, when I remove the sump, is it worth replacing the gasket around it just in case? Can't see one for a Sigma (wet sump) on caterhamparts, might drop Chris @ Redline a note if it is worth it.