My 68 reg Range Rover, with a mileage of only 24,000, experienced a recurring problem where the engine suddenly stopped while I was driving on the motorway. I towed it to a Land Rover service centre, where they confirmed that the engine had seized. The repair cost is estimated at £38,000.

reported by them
Carried out investigation into non start, found starter trying to engage. Plugged in Topix cloud found engine obstructed or blocked fault code. Checked oil and coolant levels found correct. Removed engine oil filter and found metal in filter. attempted to turn vehicle by hand unable to turn engine sized. Requires new engine, torque converter and turbos to rectify

it was serviced by them 7-8 months ago and after service it did 1400miles

Now they want £500 to put the claim to Land Rover, which may be accepted.

Has anyone else encountered this?

Tough situation.
The crank on the v6 diesel has been an Achilles heel for its entire life (have a search on here/Disco3 site) . LR of course don't recognise that for obvious reasons.
If your car has a full main dealer service history you may be lucky and they may pay for it.
38K to repair it is laughable. If you have to pay yourself, try Oval Autos near Brum. They are well regarded, unlike a lot of engine reconditioners, or if you're anywhere near Scotlandshire Disco Mikey on here is a good shout.
I wouldn't necessarily blame who serviced it, providing the oil fill plug didn't fall out. Crank failure is a lottery and servicing doesn't seem to negate the chances of it happening.
Good luck.