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Firstly, I think Rich is now inactive due to health problems, but Robin is still active and producing both kits and built units. Second, I'm not sure what the differences are between Robin's original design and Rich's build. Certainly, there isn't a LED on Robin's original circuit diagram, so I'm assuming it indicates when 'on' that the triac is being fired and the immersion should be getting the water hot.

One thing you have not mentioned - an override switch. Robin's later units have a switch to manually turn the immersion heater on. Does yours have that, so is the immersion heater itself 'working'? If you don't have an override switch, can you measure it, i.e. do you see around 15 Ω between L & N? (MAKE ABSOLUTELY CERTAIN THAT EVERYTHING IS ISOLATED BEFORE YOU ATTEMPT ANY WORK). If you can't measure that, can you link a wire between the incoming Line (Brown) wire from your fused spur to the outgoing Line (Brown) wire to your immersion heater, then power up and see if the water gets hot?

If the immersion element reads anything sensible, or if the water gets hot with the link in place or the override switch closed, then it would appear that the diverter has indeed failed. As you didn't build it yourself, I'm guessing that you wouldn't be comfortable in trying to diagnose a fault down to component level? If I'm right, you can try contacting Robin either via a PM here or via his website to see if he'd be prepared to repair your unit.

If you are prepared to work on it, then I can try to steer you through the fault-finding procedure.

Thanks for your reply Robert, I'll try some more fault finding in the morning.

Just had a thought, my diverter uses a junction box to complete the circuit to the immersion, in the style of the 'junction box' system for domestic lighting.  I could wire in an override switch in parallel with the diverter, using the same two junction box terminals, would that be OK from the diverter's point of view?

Latest update - I've disconnected the diverter, installed a manual switch in its place, switched on, the immersion is working, so the diverter has failed in some way.  I'll try to PM Robin AKA CalypsoRae as this seems to be the only avenue open to me at, the moment.

OK guys, I'm on the case ...

Thanks to Robin everything is sorted, The problem maybe due to my installation and not to the unit itself, which hasn't failed at all. I have learnt the folly of using choc blocks as cable connectors, the three inside the diverter had been melted to be almost totally unrecognisable !

Thanks to everyone for their comments, alls well that ends well . . .

Did you 'tin' stranded wires before you put them into the 'choc block'?

If so, that's the root cause of the failure. The lead in the solder flows under pressure and oozes out from beneath the screw, thus reducing the contact pressure and so the contact resistance increases. Current and resistance equals heat, so the joint warms up, the lead flows out faster... and eventually you have enough space under the screw for arcing to take place. Once that starts, you are minutes from a total failure (or a fire).

NEVER 'tin' stranded wires that will go into a screw connector.

No I didn't, I have heard that this can cause problems before, so my wires were all untinned. Thanks for the advice and explanation.

I mentioned it because we've seen it before - a couple of years or so ago.

Anyway, it's good that it failed without causing collateral damage; and even better that you are back in business.