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Turns out it was just me expecting too much from the boiler.

I was using ceramic 1/4 turn taps, so always opened them up all the way, meaning that the flow rate exceeded the 8 litre per minute that the boiler was about to kick out. Turn down the flow rate and the tap water is nice and warm.

Surprised none of you experts were able to think this may have been the cause
The cheek of you!
As hihowudoin mentions you say hot water used to be fine, but no mention of making any changes to taps etc. since then. Did you change the taps? People here assume they have been given all then facts, so based on the info given you were provided with reasonable explanations/answers to your problem. It is a lot more difficult to diagnose an issue/fault without having the appliance in front of you. The lads on here who give up some of their spare time to help people like you out because they are passionate about the job and just enjoy helping where they can. (or maybe just have just no life's.🙂) More often than not if given all the facts the combined years of experience present will come up with your answer.
Sarcastic comments like that wont get you much help in the future.
Anyway, thanks for the update and glad you got it sorted.
 
a 24 junior should deliver a 35 degree rise at 9.6 litres a minute, i have fitted alot of these boilers! 40 degree rise is a little more like the 8's
 

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