Thanks for the advice all, it is most welcomed.

If I just wanted to sell the car, I would have put it straight on ebay, or somewhere else that caters selling to the masses.
I had the heater valve done about 2-3 years ago. I remember when this went, the water emptied rather quickly and all in one go.
It didn't use more/less water depending on the heater setting. It was just losing a lot.
About 18 months ago I put a leak stopper in it, and that did manage to tide it over until recently. I had it pretty much confirmed at this time that the coolant was leaking from the head gasket, but not going inside. I put another leak stop in recently, but unfortunately is didn't work.

The oil is probably just over 15 months old, as the car was overdue a service, but the mileage I do is very low. It would have been genuine vauxhall oil in it - not sure if that is a good or bad thing.
Plenium chamber. I had to google that one, and would need to mention that one to my Vauxhall engineer.
When I fully cleaned the filler cap and neck, within a week it was really badly mayo'd up again, loads of it. This is not something that I had seen happen at all with this car before in the 12 years I have had it, and my driving habits haven't changed.
Not sure how bad a car would run with the HG gone, but it was still running pretty smoothly last time I drove it.