If you can't find a leak it could be that your pressure vessel has failed.
Once you have refilled the boiler, turn on and see water comes out of the safety valve overflow - may take a bit of time while the system gets hot and expands.
First of all thanks for the reply. I've checked the overflow pipe and it is releasing water. Also it seems like there is a slight leak coming from inside the boiler. After doing some reading around all the signs do point to having a problem with the expansion vessel
If pressure rises to above 3 bar, then yes it could be the expansion vessel. Check combi fill is turned off, you could also turn off cold mains to boiler, if pressure still rises this rules out pin-holed plate heat-ex.
If pressure stays below 3 bar - could be faulty pressure relief valve.
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