Baxi Combi 133 HE Plus loosing WAY to much pressure?

Hope you can lend some advise, I recently had the above boiler installed and I am only running 4 radiators at the moment while I renovate the flat, I am using HEP pipes and there no joins on this under the floor nor is the bathroom finished so no hot water just the 4 radiators (the others are capped off)

The issue is that when i pressurise the system to say 1.5 bar within an hour or 2 this has gone down to well below 1 bar and into the red at nearly 0, re-pressurise and the same happens.

if I then switch off the boiler at the mains it goes up to around 0.5 or just under 1 bar on occassions then when I turn it on its drops to almost 0 again yet all the radiators are on, working fine and are hot over the whole area of the radiator.

I have only done this a few times since install as I live in an upstairs flat and I am concerned that it could be a leek, however per above there are no joins and I cannot locate any leeks so far but as yet I have not lifted the floors which will be a nightmare.

Assuming that I have no leeks is there anything that I can check or test on the boiler before I commit to doing this, as if I have to keep filling then it must be going somewhere???

I have had a quick look outside but can see no signs of water from the pressure release valve which I assume is the pipe on the bottom right going outside however this could be tested further.

On a side note with this style of boiler should the pressure remain constant if only 1 radiator was connected? it would be easy to lift the hall floor to see the pipes for that radiator and I have easy access to the main junction where all the pipes meet, so I thought if I disconnected all the others and put some loops in place and only had the 1 one radiator running in theory I could work out if it was the boiler or a leek somewhere else and work my way round each radiator that was connected without the need to lift the floor??

I haver also heard others having similar issues and that perhaps it could be an expansion vessel issue??