I have a bottle jack too, fine on Tarmac, but when offroad I use the farm jack with a piece of wood to spread the load on soft ground. Perhaps just me trying not to get too muddy crawling underneath to position the jack....though on rutted tracks the bottle jack would struggle to give enough lift I find it the best tool for the job. You do need to be very careful with them though.Sorry but that is no excuse to use a farmjack, lift the axle not the body it's far safer, I always use a bottle jack under the axle. I used to have a Defender and currently drive a Hilux that's running big wheels so the standard jack is too short but a bottle jack does the job perfectly.
Farmjack's have their uses but lifting vehicles isn't one of them IMHO and before anyone says anything yes I have used them safely.