POTTERTON NETAHEAT 16/22 WITH POTTERTON EP2000 TIMER?

It's suddenly developed a fault. It heats the water in the cylinder
but not the radiators. The boiler only comes on if the HW switch
is on - however even then the CH switch is useless in activating
radiator heating. Last year it stopped working and I rectified it
by replacing the PCB with a reconditioned unit. It has been working
fine since then until this happened. If anyone has any advice apart
from getting a combi, I would much appreciate it.