I know this isn’t exactly a terrible problem but I did an 8 hour round trip last weekend in my TD5 and noticed afterwards that the engine temp was reading a bit lower than it normally does. I didn’t totally think too much of it but today I was driving for a while and, this may be in my head, but it looked like it may have dropped ever so slightly more. Like by an imperceptible amount, so it may have not actually happened. There doesn’t appear to be anything else wrong with the engine. I’ve installed aircon and a Battery Brain in the last few weeks but I’m not sure what they would do. The earth to the ECU is good.

I’m only nervous because I have a 4 day road trip tomorrow evening. I don’t really have much time to get it fixed. My next stop is France so as long as I can get it there I’ll probably be able to fix it. But I have a three hour drive before I’m there.

Unlikely to be reading lower than usual as TD5s dont have a proper gauge, it only has positions.
Off, cold, normal and too late it's cooked

So does that mean it’s good? I’m not sure where my electrical issue could be. When I had a dodgy earth to my ECU the needle would move all the time but now it’s solid.

You have checked your coolant level?

I did yes. It was quite low so I topped it up but nothing changed.

One other thing I noticed as well, I have a battery voltmeter and it’s normally read 14v but after installing the Battery Brain it dropped to around 12v. Not sure how it’s doing that, maybe I’ll double-check all the connections.

I do not know the Td5 well but it is the case that when coolant level gets low enough for the temp sender to not be fully immersed it can give spurious readings, even when an engine is well overheating if the sender is no longer in the coolant it can then read temp low on the gauge.
What is a battery brain?

A battery brain is a device on the positive terminal of the battery, it cuts the power if it gets too low to start. It makes sure you’re not stranded if you leave a light on or something. Since it sits in the path of the positive I’m wondering if the connection isn’t quite right enough or something, or maybe it’s otherwise limiting the power somehow. If I suspect it on the drive it shouldn’t be too hard to remove. I’m leaving in about an hour so no time to fix anything now.