The F&W tank is on the attic floor and the water level is certainly
much lower than the DHW level. Also the water in the DHW tank is
crystal clear.
The boilder was off before and after my check so the system was cold.
Leaking at 3 inches per hour you'd think it'd be easy to find :-(
I'll start investigating under the floor then.
Thanks for your prompt assistance!

Cheers,
Chris.