Thanks Eric
I don't seem to be able to replicate the 0.6V offset, instead I get about 250mV?
I like the idea of using op amps to isolate both sections, and it makes the hysteresis work. Presumably by adjusting the feedback resistor value, I could fine tune how much hysteresis is applied (only need about 10 - 20mV).
This afternoon, I have bought a 10A SPDT relay and a cap/resistor to fit across the contacts, which will hopefully help this first prototype, but to make the changes above will need a complete rebuild (no space on the circuit board!).