Thanks all.

I'm happy to say that Halfords fixed the problem. She's back to her old growling fiesty self, and I'm happy.

The problem turned out to be with the WATER PUMP. So suggestions to get a plumber may not have been so far fetched.

More precisely, the invoice reads "Water pump bearings worn and noisy." So they ordered in a new one and replaced it. Also "Carry out anti-freeze drain and fill." The whole thing came to 194.29 + Vat.

EDIT: Another way to discribe the problem would be a sort of sloshing, percolating noise.

Just in case it happens to you.

Peace out.