Potterton Prima 40 boiler .. what controller ?

Hi,

My boiler will only work when both CH and HW selected

I cannot have one without the other hence bills are high

I have fitted a new Horstmann C27 controller with room stat and boiler stat and switching valve

Thinking I would cure the fault.. sadly no still the same an electrician came out to check but says I have a missing wire and it wont talk to each other

He hasnt come back since
He was recommended by the chap whom serviced the boiler whom said the boiler was OK just needs the wiring ? controller sorting

Maybe I have bought the wrong controller for this boiler ?

Can anyone help or advise ?

I have got another plumber out for a full new boiler replacement quote.. but as my system is running on 15mm gas pipe, its going to cost a small fortune to renew the pipe to 22mm

Rather save when boiler does work