200 tdi (def) turbo trouble?

Whilst out the other day I was going up a steep climb and noticed I was down on power. Pulled up, opened bonnet and found filler cap had come off. After getting back on the road I have noticed a whine when getting up to max boost. The power seems to be about the same and no noticable change to exhaust colour. Could it be that I have knacked the blower? Is this common? Anyone know where to get another one at reasonable money? Should I keep running it until it lets go completely?

Was it low on oil?

You should find out why the filler came off - is it due to high pressure in the crankcase, i.e. blocked breathers, so the turbo makes a different noise, poor breathing, etc?

Not breathing too badly. I think filler shook off with all the bouncing around offroad. Not particularly tight on the tangs. I am addressing that problem with a locking arrangement. It inevitably lost some oil, but oil light never came on at any time. But the hill I went up was at least 1 in 3 so could that have led to oil starvation? Turbo makes a noise when getting up to max rpm but would also expect similar sound from escaping charge air. Obviously can't get any load on turbo when sat on the drive though so makes it difficult to diagnose.

check that your turbo hoses are all ok. If the turbo had gone i would expect oil burnage.