If you are replacing the coolant flange then no need to remove the sensor in the car, just do it on the bench. Even better if one is already fitted to the new flange.
I'm taking a break from the driveway at the moment, and having had a flashback to reading something along the lines of my current issue I've searched for "coolant sensor o-ring+a2oc.net", and it brought me straight here.
Have got the temp coolant sensor housing off in an attempt to try to get the project car usable for the return trip Scotland to Plymouth. There's been a very gentle dribble of coolant that dries onto the gearbox since before Christmas that I've been unable to trace as it is never wet - I suspect it only comes out when fully up to temperature.
As noted above, getting to the two hex bolts on the threaded rods is a bit of a pain, and there always seems to be a brake pipe from the ABS unit or a/c pipe in the way of the handle of the locking pliers to get the spring clips holding the pipes on.
The housing has come off to reveal that the o-ring that mates onto the block surface is completely flat / compressed. There is a very small mark of dried coolant directly underneath the mark on the block where the o-ring was located, which is higher up than any other marks I've seen, so I'm hoping that it is this seal that has been allowing coolant to blow past.
As noted by Graham above (
@audifan ), one of the earlier jobs I had to do on this car was replace a leaking diesel pump, so I am hoping that it was the effect of this that has slowly caused the seal to fail. A couple of the rubber pipes were replaced at the time owing to diesel contamination and going soft.
The L-shaped metal coolant pipe below this also looks a bit crumbly (and I have a brand new replacement for this), but I am going to try reassembling once I've cleaned up the block surface and hope that this has solved the primary problem for now.
Ironically, I bought the new housing when the deal price posted was posted above just in case, so it has been waiting until now to be fitted!