Pressure keeps dropping & definitely no leak

We have had this problem now for over 5 years. We have a combi boiler an Alpha CB28. Initially we thought the actual boiler was leaking (as we discovered small leaks from it) - plumbers also did extensive searches for leaks elsewhere and found none. It was advised that a new boiler should be installed. Which we did. The problem is still the same. The plumbing around the house are all drop downs with no pipes buried. The other strange thing that happens when we have a prolonged wet weather (ie the water table rises) we have to draw water off the system as the pressure rises. We have spoken to Alpha, numerous heating installers & plumbers and no one can give an answer. Can any one out there help, many thanks

Assuming its not a wind up, where in sussex are you.

Has anyone pressure tested the system yet.

Have you gone round all the rads etc looking for signs of staining.

Have you checked the valves when the system is cold.

Do you live in an high area that could be affected by atmospheric pressures.

We live in low part of Sussex where we do encounter flooding. Surface water is seen in fields next to us even after a short spell of heavy rain.

We have had all valves, radiators, pipes checked when cold/when hot - everything. We have checked all outside walls incase of leaks going into cavity wall and out via outside.

We have had the heating system isolated - the heating engineer said that this would show that there was a leak, for sure somewhere in the system. When he returned after 2 days the needle was still in the correct pressure position. He said he had never seen this before and he never returned.

It all sounds a bit crazy, but if it was my system I would still assume a leak and give it a dose of fernox leak sealer to see if that stopped the pressure dropping. I would also remove the filler loop and see if that stopped the pressure rising.