Hi, The problem could be two fold, yes the diaphram needs to be replaced if
over 5 years old and its easey [keep away from the gas side] also look at
the radiator again a simple job of replacement it could be fured up with
scale and you can get the part from the manafacturer also are you in a hard
water area ? That could be a furing up of the pipes, fit a cheap scale
inhibitor
Best wishes for hot water

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