oil in No3 Cylinder 850 T5

I have oil getting into no 3 cylinder causing plug fouling and smoke. This is particularly bad when driving in town or slowly no problem on long journeys except a little smoke on the over run. The car had done 50k and has a full volvo service history. I have done a compression test an everything seems fine, I have also pressure tested with compressed air and whilst air does come out of the dipstick very little and about 40 psi pressure is maintained in the cylinder. My thoughts are one of 3 things either;

1. valve guide oil seal
2. Head gasket between oilway and cylinder.
3. oil control ring.

Has anyone else experienced this problem and does anyone know if it is possible to replace a valve guide oil seal without removing the head???

Any advice would be gratefully recieved

Chris Knight