baxi expert req?

my faithful baxi my long hardworking baxi 105e has developed a slight niggling problem but try as i might i cant get the little beauty fixed i have a hot water problem in a nut shell if i have the heating on then draw hot water the heating doesn't come back on nor for that matter if i have the heating on if i then want hot water i have to go and waggle the hot water micro switch and waggle it again when i want the heating back on now people i do this for a living all day long but doing it at home now that's a different story now you have probably given up reading this or you have said one or all of the following in your head ,pcb=nope dhw flow-switch=nope thermistors =nope rubber diaphragm's = nope these are the things i firstly said and have checked not only with other peoplke and baxi themselves all other surgestions