Combi boiler losing pressure - tried almost everything

Hi, This is a right head scratcher! Any ideas greatly appreciated. My central heating system is losing about half a bar of pressure a week. It's a tiny back to back house, 4 rads and a towel rail. Veissman wb1b 30kw. Plastic piping. So far I've: 1. Ripped up the floorboards and run a finger over every single joint in the system, all the radiator valves, and the rads. 2. Bagged the pressure relief outlet. 3. Put a bit of cardboard under the boiler. 4. Replaced the expansion vessel. 5. Replaced the AAV. 6. Replaced the hw heat exchanger (was blocked). 7. Bled all the radiators. And I still can't find where it's leaking. Is there anything else to try? Is it time for the f4? Cheers!