check to see if it sends out a puff or two when you start it fist thing in the morning. If it does then and not so apparant during normal driving you might be lucky and only need replace the valve stem seals. I have done this on previous cars without taking off the head so you may also be able to do it in your instance (fiddly but certainly worth it). Make sure the cylinder for the inlet valve (no seals on exhaust) is at top dead centre in case you drop the valve!