I'd have followed on from this and letting the water in the thermos cool naturally find out exactly what temperature is "normal" on the guage!
Then, of course, you actually have to install the "test" sender in the engine, since they all seem to be different!
Yep did that, all seems fine with this sender so I have swapped them over. So I can confirm that the best senders to get are the Robsport and LD Part ones.
Right then, so the problem was the coil which must have been breaking down under load. I suspect that the rubbish in the fuel filter wasn't helping the situation but since that has been cleared all is fine. I have also been checking the inline filter under the car and it is clear as a whistle. The tank seems fine, I wonder if the debris in the filter was always there what I mean by that is that I received it with that rubbish in it, it could have been restocked after a return. So in the future I'll be checking fuel pumps before fitting.
Anyway, new coil fitted, relocated to the bulkhead with the Lumenition box and all is calm, happy and relaxed under DTR's bonnet! I will give it another run just to be sure but the coil is the Lumenition coil that they sell with a new ballast resistor, my logic being that if there is a problem with the optical switch then getting a replacement under warranty shouldn't be an issue as it's running the 'correct' coil.
I have also tided up the wiring and relays to the fan and Lumenition system, installed a separate fuse box for the headlights and fitted a fuse from the Lumenition control box to the coil to protect it.
This is as far as I want to go with the car. 'Dolomite's' (1850's) especially one owner early ones like this are getting rare now and I don't want to ruin it. So as there is another car that I would like to purchase the time has come for someone else to take over from me. The mileage is genuine, I have MOT's to back this up. It will be sold with a fresh MOT and as the engine is just approaching 1000 miles since being rebuilt, I will give it a first service and start a new Passport to Service book.
It is currently nestling in my parents garage so I'll wash it and take some photos and it should be advertised by the end of the week.
Feel free to contact me if you are interested.