Ariston euroCombi SX20 DHW temp variable?

I have a 10 year old plus SX20 EuroCombi - recently serviced by Corgi Engineer and declared to be in good working order.

But now I have a problem with the DHW side.

Sometimes the hot water is scolding hot, other times it is barely warm.

I am right to think this is just a DHW thermostat problem, although I am having a real problem getting the correct thermostat. The Ariston part number now refers to a part with only three connections but the one in place has four.

The divertor valve and DHW heat exchanger were changed only last year and teh system has recently been flushed with Sentinol X800 or is it X700 JetFlo as advised by the Corgi guy.

Any other ideas or should I just chase up a new stat?