In reply to gethin_allen:
Well thanks for the replies.
On a positive note, it seems like the Corsa 1.2 engine of this model is prone to generating a bit of mayo in the filler cap if used for lots of short journeys in cold climates due to condensation build up.
This doesn't however explain the residue in the coolant header tank but gives me some hope that it's not the head gasket, because having seen the palaver of taking the head off this engine with it being a chain driven cam and having an engine mount passing through the middle of the chain line, I really don't think this is one I'd attempt myself and the a garage would charge not far off the value of the car I fear.
I'm going to head out and get a few test done to investigate.