Was fiddling around under the bonnet earlier and I noticed I had a lead loose, and it has some kind of sensor on it. Buggered if I know what it is.

No running problems whatsoever - so it's a mystery what exactly it and what it does

Any ideas anybody.??

Some sort of thermistor for temperature sensing.

Has the white clip broken on one side? Can you see if it slots in anywhere.
May not come into play until the weather gets colder....

Sheer guesswork of course!

You could be right, but It's never had a prob starting and (touch wood) it runs fine. I had EGR and everything off last year and never noticed it then.
I've trawled all over the net and not come up with anything - a proper bleeding mystery!