flameport, dilalio, and Jaleak - thanks a lot for your replies.
Let me briefly explain the background:
1. This boiler was installed 18 month ago. Before the house had an old condensation boiler with tank at the loft. That was disconnected and new copper pipes installed from the new boiler to the radiator system.
2. Until this January we were using it daily (only for hot water during summer, of course) - and I never checked the pressure until we had a service couple weeks ago. The service guy has refilled the water up to 1.5 bar (he didn't tell what It was before though) and asked me to keep an eye on it. In couple days after he left I checked and the pressure was down to ~0.7 bar (nearly red sector). I refilled it to 1.5, but in 2 days it came down to the same ~0.7 and does so nearly every day.
3. Yesterday I had a gas engineer from insurance company - he examined it and said that the vessel is OK, so the leakage must be in the system. He also checked all the radiators and found green stuff (copper oxide) on several pipes coming up and down from some radiators and, although he didn't find any obvious dripping, he said they must have leaked in the past.
To my question "why I don't see any pooling anywhere on the floor and why boiler pressure didn't reduce to zero since the boiler was installed?" he replied that the leakage could be in the pipes well below the floor (~1 m down; we have an 1928 house with the base made of wooden joists on top of ground base ~ 30 inch down) and it's fully absorbed by the soil underneath.
He also said during summer when boiler isn't used, chlorine in the water can self-seal small crack/loose joints in the pipes, hence pressure doesn't reduces to 0. Finally he said that changing condensation boiler to combi boiler was not a good idea, cos combi boliers add a lot of extra pressure to the systems.
As long term solutions he offered:
1. lifting up the floor and looking for & fixing leaking pipe(s)
2. installing new pipes from boiler to the radiators through the ceiling.
1 is out of question (cheaper to buy a new house)
2 - we might consider, but not immediately and it will also be expensive and messy, as in order to connect the radiators together we'd need to drill holes through all the walls in the house and we just did a full refurbishment (wood flooring, plastered/painted walls etc.) less than a year ago.
The key wearing out is not an issue. Adding water all the time is - the result will be massive corrosion and the system being totally destroyed.
The system is in water all the time, so why would adding new water cause more corrosion?
How long has this issue been occurring?
Have you had any carpets/flooring laid recently?
Any other recent works in the premises?
As I mentioned before - the low pressure was noticed ~2 weeks ago.
The house was fully refurbished July-Sept 2014- new boiler installed by British gas, new engineered wood flooring, walls fully stripped off, re-plastered and painted; ceilings also stripped off, re-plastered and painted.
Also, we previously had leakage in pipes connecting kitchen with bathroom, please see this thread:
//www.diynot.com/diy/threads/abnormally-high-and-unexplained-water-consumption-in-house.419304/
but that was fixed by installing new pipe work from kitchen (where the main water inlet into the house is) through loft into the bathroom.
Again this was done like 10 months ago.
Tie a plastic bag over the end of the Pressure relief discharge outside, and leave for 24 hrs. See if this collects any water, to rule out the simpler things first.
Yesterday the engineer increased pressure temporarily up to 3 bars and checked the Pressure relief outside - it wasn't dripping at all, so the leakage is unlikely through that. But I will do as you suggest and report back here.
