Central Heating Playing Up

Hi all,
I am new to this forum so please be gentle.. We just moved into a new bungalow where the CH "plays games" with us.
Description of the problem: When I turn the hot water taps ON kitchen or bathroom I only get cold water, most of the times and when that happens the heaters come on even if the CH is set to summer settings. Now from time to time, and I will probably say 3 times out of 10 it works the way it should hot water from the taps and cold heaters. I hope this makes sense to you guys.
The make of CHis SAUNIER DUVAL enviroplus F28e(GAS Boiler)
Can anybody help?

Is there a motorised valve inside or near the boiler that diverts the output of heater (the fire box if you like) to supply either the rads or the hotwater heat exchanger. If so it may be sticking in the rads position, even if the gas is firing correctly.
regards M.