How can i stop my Baumatic BDW15 dishwasher going on to E4 every 2 days?

we have a BDW15 Baumatic Dishwasher that has been showing the E4 error regularly since July 2008. The machine is out of warranty and we have had the engineer out numerous times. We have only paid one call out of £98 and this is guaranteed for 3mths and we keep getting the same problem. They spend five minutes sponging out the water and then leave. The problem then recurs 2 days later. we have done everything they have suggested and do not use any rinse aid or salt and 3 in 1 tablets. we have had a non-return valve fitted by a plumber and the problem still happens. We have learnt how to sponge out ouselves now but it is hassle to keep having to do this every two days or more regularly. Why is it happening? The only one thing it could be is the water pressure as we had a pump fitted to service our shower and bathroom but this is normal pressure and the Baumatic engineer and our plumber said this shouldn't be a problem. However the pump was coincidentally fitted in July and this is when the problem started. Is this a Baumatic fault of machine that they cannot handle monsoon pumps? Are Baumatic dishwashers sensitive to pressure and can anyone suggest any other makes? I don't want to keep on calling engineers out and plumbers to waste time, if the machine is not up to it. However, i don't want to buy another make o have the same problems. Any advise appreciated!!