What could it be?

 

Could either be that the cooling system has not had a change of anti freeze for some time, or, there is some mixing of oil and water due to a gasket failure, better get it checked properly asa.

 

had a service recently and had some rocker seals changed - could that have been the cause?

 

Don't think rocker seals would cause it - sounds horribly like head gasket (happened on a previous car of mine) - needs checking m8

 

if it has air pockets in the system, overheats, uses lots of coolant etc then it's almost certain to be the head gasket. that's assuming it's the coolant your talking about and not the wiper washer bottle.

 

i thought cream gunge was head gasket, and lots of it too

my car has the coolant changed requalry and it has a light film of grey gunge in the neck, has no gasket related symptoms, i think its just due to the heating and cooling of the coolant (mine is yellow/green)

jsut check everything else too