On 19 Dec 2020 08:08:22 GMT, Peter James wrote:

Every three or four weeks I need to de-air the towel radiator in the
bathroom.

Hydrogen, liberated from the water as iron takes up the oxygen.

Does the system have inhibitor?

This is the highest Radiator in the system. A friend, who is a retired
plumber, suggested that I have a small leak somewhere in the system.

On the basis that for something to get in something needs to come
out, yes but it's not the whole story.

If a vented system is it pumping over? Even a tiny little bit just as
the pump starts (or stops) will circulate oxygenated water into the
system.

--
Cheers
Dave.