Thanks to some help from a few guys on here, I've found out that the two sensors are for the autochoke and manifold preheating. My car has a Weber replacement carb with a manual choke!
After a bit of googling and reading through Haynes, it would appear that the grey sensor is for the autochoke, so I can do without that one. I'm going to try the 'new' one but not bother wiring the red sensor (preheating) and see how it goes
Bought one of these from Euros to replace the corroded bit, unfortunately it was a bit too short, so I fitted the one I got from the breakers and didn't wire up the sensor. Seems to be working ok (now I've bled all the air out the system!)
really impressed! its a shame as mines got scratches from over the years so will never be as tidy. seriously tempted to respray it now! do you like cleaning by chance?!