Vaillant Ecotech Plus 831 F22 fault?

I've been having problems for 3 weeks now, and on the 3rd plumber, and still not fixed.
First had F75 fault after coming back from holiday. I repressurised and reignited and got hot water back. Then the magnaclean unit sprang a leak, plumber came, said it was the washer, changed it and the leak stopped.
All OK for a couple of days, same error came up, called plumber in, he tested the heating and whatever else, said it might be the expansion vessel, fiddled about and then said it was OK. Ten mins after he left the boiler failed. I turned off the heating, had hot water again, realised it happened only when the heating came on, too much pressure.
Plumber didn't come back so found a third one. He said it was the expansion vessel, would give me quote and never heard from him again.
Third plumber, seemed to sort out the high pressure, replaced the water pressure sensor, tested pump was OK, tested expansion vessel and said it was OK, after 3 hours left and the heating and water were running fine. As soon as the heating went off, pressure started dropping, dropped down to 0.4 and F22 code.
He came again, tested everything, left after 2 hours with everything seeming to be OK, again after the heating went off, same thing, pressure drops too much and F22.
The boiler is 6 years old.

Michele
October 2015