Sorry I can't help you with instructions on replacing valve - although it seems pretty accessible from photo - just ensure you isolate any power to unit as an accidental touch to 240v with a spanner or such may give you more than a tingle like 12v.
When fitting new valve ensure you apply a smear of petroleum jelly and one (1) layer of PTFE tape on new thread.
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Possum; AKA:- Ali El-Aziz Mohamed Gundawiathan
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