I had the same problem a little Mk3 1.4CL I used to own. It was the head gasket...it was letting water from the coolant get into the oil, which collected at the top of the engine...most noticeable under the oil filler cap. Its not the end of the world if its caught early. I don't know if you are able to do it yourself, but a replacement head gasket is about 10 quid from VW...its usually the labor that costs. When I had mine done, I did not own the tools at the time so it cost me 165 pounds to do at a local garage including parts.
I can't say 100% its your head gasket, but a mayo type substance in the oil always means water is getting into the oil system...the most common way this happens is a faulty head gasket. Added to the fact you report a small amount of white smoke...the symptoms are exactly the same as what I had with my old CL a few years back, so I'd suggest that is where you start.
Hope this helps!
