Ok, found it - and it does seem to be damp. But it's leaking enough to reduce pressure to 0 and cause the boiler to lockout. Easily solved temporarily by topping it to 1.5 bar, but this isn't normal is it? help
When it's turned off. I can't find a leak in the system (though some of the pipes are difficult to see), but there's no evidence of damp anywhere. Where is the water going?