Turn off the flow and return of the boiler for 24hrs and you'll be able to see if the boiler or the system.
Juts to check, you mean if I shut off return and supply to the boiler and the pressure doesn't drop - the issue is in my system?
I might actually get a chance to do this over christmas, usually a wife and toddler would be complaining about the cold
@Polygon started
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Turn off the flow and return of the boiler for 24hrs and you'll be able to see if the boiler or the system.