Thanks for the advice, greatly appreciated.
When you say blowback do you mean air? I have noticed the coolant top up reservoir froths up after a journey...
Sorry I'm so late, but I've been out all day and only just returned.
It will be air/petrol/
from your compression chamber that is pushed/pumped through the damaged segment of the head-gasket's seal around the circumference of the bore.
If you undo the cap (again be CAREFUL YOU DON'T SCALD YOURSELF if engine is hot) you will see it pumping out, bubbles etc. There will also be oily residue in the water (rainbow-ish or sludge).
I don't know what you mean by "froths up" but if this is sludge, it won't be good news I'm afraid. You may need to get a professional opinion from a trusted mechmaniac.
The thing is that if the seal has gone between the water-jacket and the chamber, the action of the piston will draw in some water on its induction stroke and then push it back out on the compression stroke. This results in fuel deposits mixing with the water and then appear in the header tank. Usually, the inside of the radiator cap will feel oily to touch if this happens. Have you ever topped it up since you had the car? If it loses water repeatedly you could be looking at a gasket or (if you are lucky) a minor leak from one of the hoses. Only visual inspection will tell.
I hope it is nothing serious, good buddy. And it sounds like the previous owner may have been the type who would buy a puppy for Xmas and neglect the dog for the rest of the year.