Worcester 3.5 Highflow no hot water.

Dear all-can anyone help?I went to look at aWorcester 3.5 Highflow today which will work in central heating as long as hot water thermostat is off, but when the hot water thermostat is turned up the boiler stops firing. I have checked programmer I have changed the hot water with another I had ,the hot water thermostat is making and breaking as confirmed by the click of the relay.The only other thing I could think of is the gas valve does it have a separate solenoid which opens in conjuction with the hot water relay?This boiler has no dhw flow switch and assume fires in conjuction with loss of heat from the heat store.This really has done my head in today am I missing something obvious?
Any advice greatly appreciated.
Sorry to ramble on..

theres not much in these apart from the diverter valve and thermostat, should be pretty easy to trace 240v until it stops. not seen one for a while but i dont think the gas valve is anything special.

Many thanks for the reply.I'll have another look.

Cheers.