hi andy, ive got the same problem with my 51 plate mondeo 3,
i'm reliably informed that it is the oil cooler that has failed, and is a pig to diagnose!
the good thing is that if youve had your head gasket changed , you should be good for another 105,000 miles lol
ive also been told that there will be discolouration and gunge for a while after.
the acid test is get a compression test done on your vehicle, and see if your comp is down at all.
this may not help you but it may give peace of mind, good luck with it anyway!