I just went to do an oil change as part of a 124k service and found that area to the rear of the sump was wet with oil (not dripping)

It wasn't like that 6,000 miles ago when I last changed the oil but as it covers a largish area I can't quite work out where it is coming from.

Could it be to do with the turbo? there is a little oil on the bottom of the turbo itself and the last time I removed the air intake pipe there was oil inside (fairly normal I believe).
What I think is the oil return pipe doesn't appear to be wet.

Could it be the sump gasket itself or some other seal?

I know that a small leak can travel around a fair bit when you're driving along so just wondered if any of you guys could point out the more common places to look.

I didn't actually change the oil - didn't seem much point if it needs to be drained to fix the leak.

Also how would you check your transmission oil and does this only apply to auto's (I have a manual (M56))

Also .... Just noticed that this service should be 126k - so I guess the service reminder comes up at 12 months or every 12k which ever comes first as it is almost a year to the day when the light first came on.

So should I service it now or wait to the 126k ?
As I change the oil every 6k it shouldn't really be a problem to wait a month or so?