We've noticed this over the last few days - DH repressurises it then the next morning it's very low again - this shouldn't be happening should it? We've bled the radiators & also checked them for leaks. My fear is there may be a leaky pipe 'somewhere' but how can we tell/will a plumber be able to identify where a leak is? Any other possible reasons?
have a look at the Pressure Relief pipe, which usually goes through the wall the boiler is mounted on. On the outside wall, it will be curled back on itself so that any escaping water squirts at the wall, and not at anyone standing underneath. Has it been dripping?
If not, and you can see no leaks indoors or under radiators, it may be a pipe hidden beneath the floor.
Thanks PigletJohn - the relief pipe does have some water below it & the ground is a little damp (we seem to think it's usually like that though)...the pipe comes through the wall then goes down vertically to end a few inches above the ground in our side passage (probably tmi but just to give you the full picture)...