I tried NFU and Flux, really expensive but they did cover my Hilux off road.
But all things considered i went with Direct line, they cover me for business use as well for £250/year, but not off road.
So all i do is carry out my own little risk assessment when i go off road, which at this time of year is very rare as even a quad has trouble going into the places i do my deer stalking.
Cheers
Richard