I have done this cant find a wet pipe, but there can be a puddle under header tank on floor

gunk in the oil filler cap can be caused by a number of things not allways headgasket failer, 1) low oil level and short journeys creating moister hence the gunk as it mixes with oil and short stop start journeys can cause this too, check for staining around neck off resevoir for dried coolant as this could be the seal gone and leaking under pressure paul

Thanks Paul i'll have a look for that later and look to see if there is any dried coolant