Bosch SMV5503 itermittent washing

My Bosch SMV5503 D/W is over 10 years old and has started sometimes not washing the dishes and has got progressively worse happening more often. It is as if it is only doing a rinse cycle. When it has finished and I open the door their is no steam and the dishes are all wet and cold, the tablet is out of the container and lying half used/wet in the bottom of the machine. On lesser ocassions it will wash the dishes properly & get hot but but I have sometimes noticed 5 or so hours after I started the programme it is all quiet and then it suddenly makes a noise and is in the middle of a wash programme. The engineer came out and did not have a clue as he said the machine was older in years than his experience. I had to pay for the call out but they sent another senior engineer a week later who arrived today and he has not fixed it but says it needs an aquastop which he is ordering. I now have to wait for the part to arrive and then make another appointment for an engineer to come out to fit it. I did not have to pay for the 2nd call out but will be paying for the parts obviously. Does anyone have any suggestions what it may be and if the engineer is likely right about it being the aquastop causing the problem.