A few months ago we had a leak from one of the isolation valves on the pump so I replaced those (and the stuck towel rail valve whilst it was drained).

But it’s not solved the problem of it losing pressure, it drops about half a bar in 12 hours then thee remainder over a few days, at which point it fills with air and gets noisy and has to br bled.

It’s not the PRV (put a bag over the drain pipe and nothing came out all week).

It’s not the expansion drum (watched the prressure as it warms up and it doesn’t move).

It’s not leaking anywere visible in the house and there’s not a hint of damp anywhere. Although downstairs the pipes are set into the concrete floors so impossible to know how they’re doing. The only other things I can think of are it’s leaking into the hot water tank from the coil, or its leaaking into the boiler and either turning to steam or going out the condenser drain?

Any other ideas? Or ideas how to test it?

When I had a similar issue I put UV dye into the heater system .

Then flashed a UV light along all accessable pipes.

Found a pin hole leak on an original solder joint. Not enough to wet anything through and it only leaked when heating was on.

Sorted it flushed out the system and put in inhibitor and no issues since.

It’s unlikely to be leaking into the hot water cylinder as you say the system get’s air into it, and that won’t be possible if the water coil is leaking.

It could be the heat exchanger leaking, which will be lost as steam or flow down the condensate drain. Can you collect from the condensate drain over a period of time when the boiler isn’t running?
Does the system leak at the same rate if the system is running or if it is all turned off? If it leaks noticeably faster running, that could suggest the heat exchanger…

Running pipes in concrete floors can be ok, but they need to be wrapper. If not they will develop pin holes over time, and leaks will be very hard to spot until the leak is very bad. Can you fit a couple of valves to completely isolate the downstairs heating loop? If so, that can confirm if it’s that loop that is leaking, or not.

The leak is enough that I would expect to notice it, and that it wouldn’t just be masked by evaporation when it’s running. I’d guess at the boiler or in the concrete floor. When is the boiler service due? They should check the heat exchanger…

Good luck!

Did you re-pressurize the boiler?

Not a plumber so just a suggestion … but we had similar after bleeding a radiator, the pressure dropped and the heating and hot water wouldn’t work. A quick goggle said to re-pressurize the boiler so I looked at the manual that came with the boiler and it was easy, a tap thing underneath turn until it’s (the pressure gauge thing) in the green, turn it back off and it’s been fine since.