A bit more progress
So the built module has been tested on the car and installed into the OEM box. Instead of plugs mounted on the ends like the OEM unit, I have left them on fly leads because not only is it easier, but when I took the original unit apart to salvage the plugs, the way the plugs connect to the board appeared to be a weak spot. One of the solder joints was dry as though it had been fatigued, and was probably why that particular unit was intermittently faulty.
So here it is mounted inside the box. I decided to install it on its side because the wiring was just easier to deal with.
Next step is to surround it with potting compound, but I haven't got any yet. I want to test it a bit more first because, as previously mentioned, once potted there's no going back and I only have one thermocouple module at the moment. I will pack it with some heat resistant material called accoustafil (which is actually used for exhaust packing) and find a lid to cover it as a temporary measure. It can then be bolted in place of the original unit.
Or maybe I'll take a risk and just fill it with potting compound! It's not as if the components are particularly expensive by comparison with an OEM ECU and I have another 10 circuit boards anyway. Yes, I ordered 10 and they sent 11.
What I don't have is another 10 OEM boxes!