chances are its something little and its never a coincidence when something goes wrong, weeks after the area near it being worked on 
Well, the mechanic had a look with me, and he said it isn't blowing, so he doesn't think it is the manifold or gasket. He said there is often oil in places on the engine, but he is baffled as to where the power has gone, as it only seems about in the 160's bhp where it should be 200+ for an AUM
I get the rough idea of where it is, but cant think of anything there that would leak boost, didnt you fit a TIP recently? have you checked that ?
Yes I had a tip fitted, I don't think the tip is the problem, but will ask the mechanic if he can see a leak there. The pipe or metal bar (whatever it is) is kind of inside the back of the block a bit. There is also oil over the heat shield that covers the pipe that connects to the pancake pipe from the charge pipe.
Could it be coming from the block?
AUM? mine always leaks/is covered in oil. no performance loss issues tho
It is not that part on the pipe that is leaking on mine, because the soot/oil is on top of the heat shield that covers the pipe, so it is coming from somewhere else.
This may sound a stupid question, but if you don't know then always ask, so I am told

But, where is the sump plug? if this was leaking could it cause a power loss. I ask because when I got my car back from VW last week for new coilpacks recall, they said I needed 1/2 a litre of oil ! Now I only had an oil change done about 6 months or so ago as part of a service, so it must be going somewhere?