Also, if you had a blockage somewhere then it would eventually dump some coolant into the overflow/on the floor

Thanks Kermit. Why had I not thought of that! I am gonna stick with the thermostat as is, as thats how the original engineers designed the cooling system. I live in a frost pocket too, so I do need the heat to defrost the cab. Having replaced the head gasket 2 years ago, the engine block was thoroughly cleaned and tested along with the head. Since then I flushed the system last autumn, so I am finally getting to the conclusion this is sensor related. I have just bought 2 x NOS sensor from PB Blanchards, so I'll fit that and remove the recent britpart. Will update when I get to the issue. Thanks again for spending the time thinking and then posting. Its really helpful as a stumped long term DIY mechanic.

As above @kermit_rr re thermostat.
I often say for older vehicles you can not beat having an aftermarket capillary type temperature gauge, keep original for looks and fit it somewhere else. They read in numbers/degrees and no electrics to bother with other than light. Hope you pass on what you learn here on the forum, we need more members doing that. Cheers.

There is huge variability in sensor calibration. I ended up going through my stock of old sensors until I found one that matched the gauge

Just an intermediate update. The infrared thermometer arrived this afternoon. Cylinder head 79 degrees at the back 78 degrees at the front, top of rad 68 degrees. Fitted new Britpart temp sender - this sent the dashboard temp gauge just below the red. NOS Smiths temp dated 1977 sender fitted - this sent the dashboard temp gauge to above N, but well below the red. Intermoter NOS dated 1980 exactly the same as the NOS Smiths. To complete the picture, I have just ordered a brand new Smiths. Sadly, I am no electriciation as I'd likde to check the OMpS on each. However, I am pleased with the infrared thermometer results - but if I can I'd like to get the dashboard gauge bang on N as it had previously been. What I did note is the Britpart is short and dumpy and clearlt fits closer into the cylinder head, whereas the Smiths and Intermoter are both longer, which I feel must sit further into the water jacket/channel. When I fit the new Smiths, I will report back. At least I have a few spares to now carry.

What's the voltage regulator output like? 10v steady?

Yes. Having bought NOS stabiliser and then a new one (neither worked), I was recommended on another thread to purchase a modern equivalent from ebay. For the first time in years I had an accurate fuel gauge, yet the temperature was in near the red with the Britpart equivalent. I have a Smiths and Lucas on order. I am running the 1977 NOS Smiths temp sender at the moment. Needle sits above the N, so you can clearly see the N, yet it does not stray anywhere near the red. I will play around with the new Lucas and Smiths and report back. Previously, the needle sat bang on the N. Today, I had the bike carrier on and after every large hill I was dashing out with the infrared temp reader. CH constantly at 79 to 81 degrees. Top of Rad 70 degrees, bottom of rad 40 to 42. Covered only 8 miles - yet it was pretty consistent, so I am now hugely confident this is just down to the electrical resistance in each manufactures temp senders. Also, each is a slightly different shape, with the Britpart having a stubby nose when compared to the Smiths and Intermotor. Will report back in case I get a different result with the modern Lucas or Smiths. Once again, thank you. This long running saga is nearly over. Just the front swivels to go this winter....