Diplomat dishwasher – water not getting hot (error E3)

Hiya, can anyone help? We have inherited a Diplomat dishwasher with our rented flat. We suspected it was not getting hot when it was failing to remove butter from knives. Having finally secured a copy of the manual from MFI Extracare (thanks Dave!) it turns out that yes, the lights that remain on after a cycle are an E3 error, saying the water could not be properly heated. Why might that be?

This may be totally unrelated, but we also have a Diplomat washer/dryer and we have wondered whether the water is getting hot in there either as it isn’t cleaning things very well… but maybe that’s just a coincidence.

Any advice would be much appreciated as it’s out of warranty and there is no service agreement (and I’m not sure to what extent our landlady is going to want to pay out…!). It doesn’t look very limescaley in there or anything. Thank you!

PS. Ours was smelly too (see other post) but after cleaning the filter and installing a freshener disc, it’s fine.