Update. I worked out which unit I needed, and got one delivered. Following advice from this forum, I bought a party pack of jubilee clips from ScrewFix, loads of different sizes for about £11. Useful to have around. I changed the air filter at the same time, so I took off the air inlet passed the resonator box, if that makes sense?
Getting the old clips off was a bit of a challenge, but as I had numerous new ones I was not trying to keep them. I struggled to get the lid off the air filter, but plus gas and mole grips sorted that, but someone had broken off one of the lugs so the lid only is held by two scres and not three. So if anyone has a lid that I could buy I would be grateful. I will try and find some bolts to replace the cross head screws, stainless steel would be good.
Replacing the theromstat was straight forward, I remembered to block up the hole in the clutch cover!
All back together with new anti-freeze, and the fan came on as soon as I started the engine, as I had not pressed home the sensor connection firmly enough. After that, panic over the engine warmed up the the 12 o clock position on the temp gauge, and the fan cut in. Deep joy.