you probably know this already but.....
Bummer dude. Check the basics, fuel, spark, air. I've had electric fuel pumps die suddenly. I now carry a spare, don't know if you're running elec or mechanical. Fuel filters get clogged. Check spark by pulling a plug and turning the engine over. A friend had problems with his Rover running like crap, didn't totally quit running though, just because his K & N filter was sooo dirty. Checking these things may not zero your problem right off but if you can eliminate 2 out of the 3 then it makes finding the real problem easier.
Jeff
1964 Series 2A SW, LHD mostly stock, often runs!
1991 Range Rover Hunter