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diamondgas
Are you are sure the pump is running correctly (remove the bleed screw with a cloth underneath and put a small screwdriver inside to feel hear the impellors turning) as mentioned before try user control stat? Tricky one this.
If the pump is not running correctly you will still feel heat in the flow pipe (By the boiler) just not enough to take it away from the boiler and it will shut down on the temp stat.
If the pump wasn't working the OP would have no hot water armourer ...