The other day I noticed the pressure in the system was rather low. Topped it up to 1 bar, but over the next few days it slowly fell back again. I assume a pinhole leak somewhere. No sign of an obvious leak on the CH system (no damp spots on the ceiling, for example), although much of it is inaccessible. I've checked the boiler pressure relief blow-off pipe outside and it's dry, so probably not a leaking relief valve. Any other suggestions?
I see that there are additives available for sealing such leaks, for example Fernox F4 http://tinyurl.com/pum2e54 but there are other makes. Are these generally OK to use, or do they cause problems over time? Is the Fernox one the best or can anyone recommend a better alternative? I would add it through a radiator bleed valve, so it needs to be in the right sort of applicator for that.