diverter valve is stuck, i think they give priority to the water and then the syncron motor switches the valve to the mid (heating and water) position or to the heating only position to give heating when its demanded, very common problem, will need either a new syncron motor or possibly a new head or in a worst case scenario a complete new valve and head.
turn the water off and heating on at your programmer, make sure your room stat is turned up high if you have one and if your husband finds the valve, which is normally near to the hot water cylinder and pump, they have a little lever on the side, if he uses his finger to manually move the lever over and the boiler then fires up then yes the valve is stuck. you can lock the lever in the heating position to give some temporary heat by wedging a screw in the gap between the lever and case to stop the lever springing back to the water only position. i'm not brilliant on the old style systems as ive been brought up on combis and i know that theres one way my father temporary locks the valve so you can get heating as long as you have the water on too, trouble is when the water reaches temperature the heating goes off too.
to help identify the valve they are commonly made by honeywell or danfoss and have a silver metal case or a white plastic one, they are about 6 inches long and rectangular. be warned that the valve is 240 v so caution is needed.
i reccomend you call out a heating engineer asap as your husband is going to need knowledge of plumbing and electrics to change or repair the valve.
i think gasman who uses this site lives up your way somewhere but i'm not sure how far he is from sheffield.