Since the boiler went out of warranty last November I was suffering low pressure on the boiler, it used to happen on a monthly basis, it went to weekly, since the last service engineer came it lasted a week. I topped it up yesterday morning and it needed topping up this morning. The engineer seems to think it's a leak, but there are no obvious signs of leakage unless in under the ground floor boards. The engineer has increased the pressure using a foot pump, replaced two valves, but not cured it.