The fault most certainly lies with the thermostat not your boiler, it sounds like the control board in the thermostat has failed.
I would think based on this fault, its the HCW80 which is the problem and needs replacing, instructions for removing it can be found here (its reverse to installing it). You can buy a replacement HCW80 unit here for around £60.
No the HC60NG is unlikely to be at fault, as its receiving the HW commands, so it should just be the transmitter not the relay.
No problem Jenny, if it doesnt please come back to me, but from what you have described it should fix the fault.