Temperature issues / thermostat replacement - advice

[JudeSunFlower]

Hi - my temperature gauge started moving more than usual yesterday. Thankfully it wasn't moving beyond the 11:00 position but did seem to indicate that the engine was fluctuating between a cool and normal operating temperature. I think this might mean my thermostat is faulty. I wonder if anyone can help me with answers to the following questions:
1) does my diagnosis sound plausible?
2) can anyone point me at a step by step guide or video explaining the process for replacing the thermostat? I'm not an experienced mechanic and if the thermostat is where I think it is on the engine it looks like a tricky job!
3) is there anything else I should be considering?

We're hoping to drive it to Spain in a couple of weeks and coolant issues give me the fear!

PS - I did find some excellent vids on bleeding the system but nothing on the thermostat.

Thank you all in advance.

[g8dhe]

I would suspect the sensor itself first, before the thermostat, check its connection its a single green wire at the front of the engine, accessed from the drivers side, it plugs onto the sensor using a spade connection, it can come loose, corroded or the sensor itself can fail/intermittent.

[JudeSunFlower]

I would suspect the sensor itself first, before the thermostat, check its connection its a single green wire at the front of the engine, accessed from the drivers side, it plugs onto the sensor using a spade connection, it can come loose, corroded or the sensor itself can fail/intermittent.

Thank you. I just popped out an checked the thermostat connection and it seems fine. Gave it a light wiggle with everything on and nothing changed. I also tried a number of other things that may help clarify the situation:
- running up hill - temp increases
- at speed on dual carriageway (c70 mph) - temp decreases
- stationary - temp increases and stabilises but not a full operating temp
- engine off after period running and then turned on - temp warm initially and instantly cools when engine running
All of the above were done during a 30 minute drive...

[g8dhe]

The Thermostat and the sensor are totally different things, on a diesel the Thermostat is on the passenger side I believe, its not electrically driven/connected at all.