Hi, it may be worth going round the rads and bleeding them, this can sometimes be the cause of pressure build up in the system due to the reaction of the water on the metals in the system, but be aware of how to top up the pressure at the boiler before doing this because if the presure drops below a certain level the boiler may not start up.
Suggest get someone to watch the pressure as you bleed the rads,but don't let it drop below 1 Bar.
Alternatively, find a drain tap on the central heating pipes, usually at a low point in the system and drain some water slowly into pan by turning the small square nut on the valve.
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