Today we replaced the fuel burning heater with a second hand unit from the motherland.
Ebay global shipping rort confiscated the first one for reasons unknown, but the second identical unit made it through. Wasn't a huge job. Drop ~10L of coolant, replace heater and replace coolant.
A significant amount of time was spent dealing with a spinning rivnut in the chassis, but most of it went in search of a 10mm socket that disappeared into the twilight zone. Over an hour pulling covers off to come to the conclusion it was stuck somewhere awkward, and 5 minutes with a borescope and flexible magnet to locate/retrieve.
Then put all the covers back on with a liberal application of profanity.
I'm not quite sure what to do with the rivnut. I can't get it out. I'm considering silver soldering it in place but then I thought about a tack weld too. It's an M6 nut, but must be full of corrosion so when I screwed an M6 bolt into it the bolt wedged and when trying to remove it the nut just spins. It's holding the bolt now although the bracket isn't tight against the chassis. I'll have to sort it at some point.
Part HN1 : JEC500820 - Land Rover Heater | Jaguar Land Rover Classic Parts
It's a bugger to get to if I have to drill it out and replace it.

