leaking overflow from combi boiler

Ectophile

Having had some dodgy pressurised central heating in the past, I can see a number of possible causes of the problem.

Pressurised heating systems should normally be around 1.0 bar when cold. This may the rise a bit as the system heats up.

Anything over 3.0 bar is too high, and the excess pressure will vent to the overflow.

If your system is at 1.0 bar (or less) when cold, but hits 3.0 bar as it heats up, then the expansion tank has failed. Get a qualified plumber to fix it.

If the system is well above 1.0 bar when cold, then it's been over-filled. With the system cold, stick a suitable container under the bleed valve on a radiator. Open the bleed valve a bit and let water leak out until the pressure falls to 1.0 bar. If the heating has a drain valve, it may be easier/less messy to use that instead.

If the pressure keeps rising even when the system isn't in use, check that both the taps on the filler pipe are properly turned off.

macman

Exactly. I'd suspect that the filling loop is still allowing water in, so it's continually trying to repressurise.