Hi Mandate,
In the end I didn't replace the switch. Although they were available from the link I included last time - the store is/was at Stockport. The metal actuator arm looked longer on the replacement, but I reckoned I could have swapped that for the original without much grief.
The source of the problem was the plunger arrangement (on the other end to the diaphragm) of the diverter valve which operates the switch - it was passing a very small amount of water through its seals and off the end of the plunger into the plastic housing of the switch and, the way the housing was tipped caused some small droplets to get into the switch (almost looked like condensation).
I got a servicing kit for the valve and diaphragm assembly (not overly expensive, and honestly easy DIY stuff - if you need to take it to bits though, it has a couple of springs which like to ping out - you get replacements, but you don't quite know how they go!).
Once that was serviced, with switch removed, I dried it out, added a v.light squirt of WD40 into the switch and clicked it loads of times and found my multimeter checked out the switch to be 'making and breaking' just fine.
Put it all back together and it's been fine ever since..........
Be aware, if you take off the valve, it will drain the heat exchanger above! Depress the pressure relief valve to drain it all off first!!
