Thanks for your reply.
Yep, the pressure consistently drops to about 0.3-0.4 bar with nothing isolated and the boiler turned off. It's slightly less than 0.5, but I can't be more accurate than that.
The system has been turned off the last few days for all but a few hours a day and the pressure has never hit zero.
The cold water tank is about 3 metres above the hot water tank, which if my memory serves me correctly is 0.3 bar (10m water pressure = 1bar?) so I did wonder if that might point to the coil.
I'm loathe to turn the hot water on when the pressure's only at 0.3/0.4 bar. Is it safe to do so without knackering the pump etc?
Thanks