Heatrae Sadia Handy 3 not heating ?

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Hi folks I have one of these in an annexe which we holiday let. It is not heating the water ...... here is what I have tried / tested so far..

turning the water flow rate down and the water out is lux warm at max setting just before the low pressure switch kicks in.

the element measures 18.2 ohms (3kw... so that is about right)

there is 240v on the element ?

Suggestions please ?

cheers