Just an update on my problem. Oilhead mentioned "failed expansion vessel" and, after genning up on the subject, I discovered that there was no air pressure in the EV and by tapping it, it was obvious it was full of water. It was a simple job to drain and take off the EV, pump out the water and repressurise to 3bar. Glad I didn't have to get anyone in the house as our wee village now has several Covid cases after someone came from Glasgow to visit a friend here. Thanks again to all (especially oilhead) for their responses on the forum.
There was no need to drain or take it off, you just needed to add air to the correct pressure, which would then force some excess water out via the PRV.