I shall post on the offchance that someone happens to have a hint, tip or previous knowledge of what might be wrong with my sister's boiler, before I go an have a look tomorrow.
It's a Saunier-Duval ThermaClassic F30E, currently exhibiting error F9 on power-up ('Water pressure sensor fault')
It's just had the 3-way valve replaced as it had aged and was leaking (quite a lot!) with micro-holes. So there may well have been water ingress into various parts of the system..
The water pressure does seem to be read, as the boiler briefly (for a second or two) reports the reading at power-on, before going into a constant 'f9' state. (Adding water to the system increases the reported value if you power off/on)
So... I'll probably try remove/replace the water sensor itself. But if that's not the problem, is there anything else worth trying?
Cheers!