Central Heating System losing pressure despite replacement parts and multiple visits from plumber?

[doitmyself]

Yes is it one of them ? They’re a proper pain to relight when they go off.

Yes, it's one of them. Rayburn is owned by Aga Rangemaster and both Aga and Rayburn products are made in the same factory.

 

[dan_the_plumber]

You’ll probably find that your expansion vessel(s) need re-inflating with air.
Also top-tip: Find pressure relief discharge pipe outside house, fit party balloon over pipe. If balloon fills up with water you’ve found your leak.
Your system has no air in expansion vessels, so no room for expansion. PRV ‘blows off’
You need to re-inflate expansion vessels and may need new PRV.