Thought I'd best post to break up the fight!!!
Cheers for all your help chaps - it would appear my shower unit could be the problem and I shall investigate forthwith.
I'll let you know the outcome but I suspect you've hit the nail on the head (or the ball on the ...erm...cock)...
Yep, it's the cold storage cistern, not the F&E tank. If I understand you right, water's getting into it through that pipe in the corner pointing down into the tank from a mixer valve? This must be the shower unit in bathroom, as I don't think there's anywhere else in the house the feeds are...
Hi - as I'm sure will quickly become apparent, aside from the basics, I know very little about plumbing! Therefore I am appealing to you beautiful, full of wisdom, people (flatter flatter) to help with a strange (or maybe not) problem.
The overflow from the cold tank in my loft has been...