MartinC

So, with safety override in place, kiln errored at about 200oC. With it unplugged, kiln has now fired twice without error so Im think that's where the problem lay. It is superficial as the safety override now comes from the controller so I think the mystery is solved. Thank you to everyone for your engagement and help.

Looks like a safety override. Kiln errored with one hour of starting. Have disconnected it and am trying again

Have called Kilns & Furnaces to try and get a diagram

Yes, the door switch is operating as needed. Opening the switch turns off the controller. Plus, it is a Stafford controller which restarts in case of a disruption to power so that isn't the problem as the controller is still on when the kiln fails. The black box looks like it might be bespoke to the kiln brand hence no part number for a third party supplier.

The front is visible on the front of the kiln. It has a red light that comes on when the kiln is firing. It looks as though this box is what receives the signal from the controller and then transmits to the contactor to open/close. If the door switch is just that (for safety) then this could be the source of the problem. It will be old and although not drawing a lot of power, might be sensitive to the ambient heat of the kiln (or maybe even the cold when kiln won't start at night). It will need testing when the kiln fails

No keypad, external controller

Yes, it's wired from the controller plug and the thermocouple and outputs to the contactor

Here is the mystery box between controller and contactor. It's helpfully labelled relay so the problem could be in here.

What about this mystery box that is wired from the controller and to the contactor?

Following from the post above from High Bridge Pottery, the door key slots into this and turns the rotor, closing the contacts. I assumed, therefore, that this was then just a door safety switch. How does it work, if it is a relay?

For example. I set the kiln to come on Thursday night. The kiln didn't start (not the first time it has happened). I started it Friday morning and it started fine. It got to about 1000 and then stopped firing. The common failure is it gets hot then just stops. When it stopped firing when it got hot I could hear a quiet click. I have two controllers, both electronic so assume they don't have any moving parts to "click", and the fault was the same with both . It was not the heavy clunk of the contactor but something fainter. Perhaps it was the contactor making a different noise but I can't be sure. My wife was with the kiln on Friday and said she could hear any clicking. I looked inside the door switch (attached) and it looks fine. When I changed the contactor I got about 3-4 firings before the error started again. Honestly, I'm perplexed. (Having said that, I swore the quiet clicking was coming from the controller but that cannot be the case, surely?)

Yes, it's Bill from Sedgefield I know. He's actually retired and his assistant now runs the supplies side from Darlington. Bill still does repairs though.

Yes, I know one who lives close by. I think this may be it now

Unwired the contactor, everything looks clean, no burn. Door relay also.

Yes, I think that's basically it