Diplomat ADP8132 barely starting cycle

Hi, I wonder if you can help with this one…
I have a Diplomat ADP8132 about 5 years old. It’s not completing its cycle. The symptoms are as follows:
It starts ok and begins the cycle by draining. It fills a bit then drains again for the second time. All normal so far.
Then it fills again and instead of then bursting into life it stops, leaving some water in the base of the unit. At this point there is no error code. If I start the process again and leave it for half an hour or so and then open it, warm air comes out as if it is in the middle of a drying cycle. If it deigns to give me an error code, which is a bit random, it is E3 Dishwasher unable to heat the water. But the element clearly works.
I have flushed out the tube in the drain sump (the one with the rubber flap) and reset the machine (power off, on then 4 times on the programme button) all to no avail.
The only other difference is that on previous occaisions when it wouldn’t start the properly, it would stop after the second drain cycle usually with an E2 error and not leave water in the machine.
Any ideas? Thanks in advance