Vaillant Ecomax boiler - Fault Code F22 - Please help?

Major probs with my ecomax boiler. Over 2 weeks ago, when in shower noticed problem whereby hot water would run cold, and boiler would display F22. Water pressure fine, and reset button resolved issue. Didnt happen with every shower, and only once a day. Have care plan with certain energy company, and have now experienced 9 callouts in 2 weeks. During which, i have had....new water pressure sensor fitted, 2 NTC sensors fitted, new plate hear exchanger fitted...and still F22 displays intermittant. Engineers mentioned that he isolated cold water supply into boiler but as there was cold water then coming out of my mixer taps when he turned hot water on that it wasnt the boiler - it must be the blending cartridge / thermostatic valve in shower. He left having turned controls to a position he said isolated the prob, and told me not to use that shower for 1 week to prove that was problem, except now the same F22 problem has happened when running upstairs bathroom bath tap (this happened once before and once in downstairs WC though engineer ignored that fact when diagnosing shower) Again, water starts hot, turns cold, and F22 on boiler. This cant then be connected to thermostatic valve in shower then!!?? Please help me