Yellow cream in header tank

Depends on where gasket goes whether it mixes with oil or not .A sniff test (block test) will tell you if head gasket leaking / blown.

In my experience (which is much less than Whittingehames!) if it goes away after a long journey then you are ok. My current car had some of this when I bought it - since then 10,000 miles later it's still ok.

I had a fiat which blew a headgasket - there were water drops on the dipstick, the oil looked like mayonaise and there were clouds of white smoke/ steam coming out the exhaust. I guess not all failures are as obvious.

Good luck