I have a British Gas contract and they are being incredibly helpful, have replaced a couple of obvious parts (pressure relief valve, pressure switch (or whatever it's called)) but the boiler is still losing pressure incredibly quickly (ie drops from 1 bar to 1/2 bar after say 6 hours of central heating being on) and therefore a lot of water must be being lost very quickly somewhere. There are no obvious signs of water loss, no underground pipes, no hint of leakage on the downstairs ceilings from upstairs pipework even when the heating is only on upstairs (where the boiler is, and a diverter allowing water into the downstairs circuit is shut off) and therefore in theory the water loss must be upstairs somewhere. Could it somehow still sneak through the pressure relief valve? I know it's a ridiculous request but anyone any suggestions? B Gas are saying after 4 visits to call them in the new year as by then something must have started to show but there is very little hidden pipework...