The boiler is a Glow Worm Flexicom 18 SX. To set the scene, there's a slight leak somewhere in the heating. No signs of leakage visible anywhere, and I've even had the hydrogen leak detection boys out three times so far; they can't find it either! It's obviously opening up when the system heats up, and closing when it cools down. I may try a power flush next; SOMETHING must work... To keep the water loss down, I run the boiler at low pressure - not above 1.0 bar cold. It dropped from 0.9 to 0.6 over four months during the summer, when I was running only the hot water for an hour a day, but with the heating going as well now the loss is more - about 0.1 bar per day. It would drop to 0.5 bar previously, as per the spec, before I got the F9 code, but now I get F9 at 0.7 bar on the display. Replace the pressure switch (easy and cheap)? Call an engineer (easy but not cheap)? Does anybody have any idea what might be wrong?