Alistair, theoretically, you should be able to run your Land Rover on a 50% mix, but the Lucas CAV injector pump on the Land Rover diesel doesn't like anything much more viscous than diesel, because it uses the diesel to lubricate the pump.
To overcome this problem, especially in the cold winters you get in Scotland, it would pay you to fit a dual fuel system with a heat exchanger in you waste oil tank, and I would still cut the oil with diesel fuel or heating oil, you could reduce the amount of diesel in warm weather, it would certainly save you a lot of money in the long term.
Kind regards...Ron