Boilermate bp heating & hot water

I have a boilermate bp and am having problems with running the hot water and heating together. They will both start of ok but as soon as the hot water reaches temperature the boiler cuts out all together and wont run on heating only unless i run off hot water or put hot water off at programmer.
It has been checked over by plumber who replaced 3 port valve and says flow and return all correct and electrician who checked all wiring and says all wired correctly, but is going to change the cylinder stat to see if this will solve problem, any ideas if this doesnt work?

How long has the BP unit been installed?
And was it ok?
Mike
MGC/H2O
Norfolk

The house was a showhouse for about 1 1/2 years before I moved in 6 months ago, didnt notice to start with ,but in the first winter when running both together the problem occured.

Right
I would say the unit and controls are not wired correctly.
A decent sparky with all the information in front of him should be able to confirm this.
Is the unit hot?
If it is then the boiler and control stat for the store are ok.
The system you have by your info is a Y plan.
When heating is called for by the programmer and room stat.
The valve should open and switch on the pump and boiler.
It is a wiring issue, if the 3 port valve is ok ?
Mike
MGC/H2O
Norfolk