You need to do a bit of work to check the heater is the culprit, or if it is the power not getting go the heater, fo some reason.
You could do worse than startiung with this:
http://www.bekospares.co.uk/dishwasher/dwd4310/flow-through-heater-1800w/product.pl?pid=949201&path=61208&model_ref=3533738
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June 2015