Oil overfill/oil pressure warning ????

Posted this on the B6 forum only to be informed I have a B7 so re submitting here. Thanks all.

I have a 2.0L TDI 2005 so think its a B6 (Now known to be B7). The engine code is BLB so hope this helps.

I noticed a couple of days ago the engine seemed noisy on start up in the morning. Once warm it was fine. So I checked the oil and it was down to the minimum on the dipstick. I got a 1L of oil and put it in. I didn't think that 1L would take it from min to overfilled so didn't check just drove home. as it was late on Sunday evening the roads were empty and the car was fine. The following morning after driving for just a min or two the oil pressure light came on. This came on a couple of times and went off and once I reached the motorway all was fine and didn't come on again. On the way home this was repeated. Once home I checked the levels and the oil was past the max. It was up to the flat bit of the dipstick past the bobbles so I think this is overfilled. I took the car today to get an oil change as it was due in 1500 miles and I still have the same issue.
I have noticed if I keep the revs above 1800-2000 then I have no issues. Once below about 1800 the oil pressure warning comes on. The car starts, idles and accelerates fine. The turbo spins up fine and I have suffered no loss of power at all.
Having read a lot of info now about the oil pump being chain driven and this causing issues I don't know what to try next. It seems it all started when I added the extra litre of oil.
The car is at 143k and I have the oil change done regularly and the car hasn't skipped a beat in the year I have owned it.
What peoples thoughts?

Thanks in advance for your help.

Thanks all, I really did hope it was going to be something to do with the addition of the oil. I am not driving it anymore and will have it looked at to assess the damage and get a quote. Fingers crossed.