Aidan wrote:
Probably pumping over, water going down the cold feed and back up the
open vent. Check the layout of the pipes, if there's a lot of pipes and
fittings between the CF and OV connections,

Depends on what constitutes a lot of pipe(s), the layout is roughly like
this

OV.
|___________
CF |
_____|_____|
| |
| ---------
| | |
| | |
| |Boiler |
|
|
|
Pump

The flow comes out of the top of the boiler enters a T-joint, continues
straight up for 50mm before turning horizontally. The pipe then runs
600mm horizontally before turning upward again. This is the overflow
pipe and it goes through the ceiling to the cistern. This vertical run
must be over the 1m min height specified in the installation manual.

The other leg of the T-joint proceeds about 300mm before another T
which is the 15mm cold feed. This run is obviously longer than the
150mm max specified in the manual. It sounds like this is the problem?

or if the CF connects to
the boiler return and the OV to the boiler flow, then this is probably
the cause. The plumber may have noticed and didn't mention it because
he didn't want to get involved in sorting it out. It needs to be
sorted. The oxygen being absorbed by the water will cause internal
rusting of the rads. Had any rads leaking?

Not yet, and most of them are pretty long in the tooth.

Thanks for the info

Kind regards

Ross