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.
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. In his day he would have solved it with the addition
to the system of a "leak fixing additive". But he does admit he is totally
out of date and things might have moved on from this.

No they haven't.