It's actually the black one...
I replaced it just before the stat and tank though so it's only been in there a number of weeks.
That would explain the missfire, but would a dodgy inlet gasket also contribute to the cream under the filler cap?
Thanks for the reply!

Hmm, just bought an inlet msnifold gasket ready to replace it, and it's a green one...

Replaced 1.8 K-series inlet manifold gasket after slight loss of coolant when the vehicle is stood after use. Still seeing slight loss of coolant & lumpy starting; no emulsification on oil filler cap. Any possibilities apart from head gasket? It was done 12-15,000 miles ago (at 44,000 miles) just before we bought it.

If the head gasket wasn't done correctly, then it'll fail again.
The exact HG replacement procedure needs following to the letter, which includes measurements of the liner heights and selecting a gasket that is suitable for the liner measurements taken.
In my experience, very few normal garages know how to measure the liners, or know what the correct HG replacement procedure is.

Hopefully it's not a HG, but it can't be ruled out completely.

Has the coolant reservoir been released? That's often a site for coolant loss.
The water pump can also weep a little coolant, which is actually pretty common, there's even a little evaporator built in to the pump, to evaporate off the leaked coolant.