honeywell valves ? take the metal cover off and have a look as it motors over there is a little brass paddle that you will see moving and it then activates the micro switch you can twist it a little bit to get it to touch microswitch if its just not quite making it .I think I’ll wait until the summer when the heating is not used, drain the system down and just replace both valves complete with heads etc.
As for both what has prob has happened is HW one went first so valve will have been opening and because you have been using the heating for months now the cylinder will heat up . Now the heating one is playing up it is showing both being faulty