I had this fault a while back, and went to the board before I knew the the tacho was the most likely cause. I changed one of the relays in an attempt to sort it before Dave put me right with the tacho advice. There were two relays controlling the heater, but switching L & N as I remember. If one were to fail it wouldn’t give 116v at the heater, but under no load conditions the 116v may just be a red herring being half of 230v. (Was that reading taken across the heater terminals? If so it implies some current flowing.) Further than that I can’t really help. The rear module was a devil to take out – wiring holding it tight – but beyond that it’s easy enough to access relays which were 12v as I recall.
Graham