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Hi this is most likely to be your plate heat exchanger scaled up or very dirty
Hi again here is the manual for the boiler
You will need to head firstly to page 42 and have a good read through before going further.
you can clean the plate heat exchangers with Fernox DS3 if its scale or a good boiler cleaner if its sludge.
Personally i dont clean them now and will only replace as i cannot guarantee a good repair.