Hi Andrew,
Rough shooting is a slippery thing to get hold of and like Cazzie says there's a very high demand for it and not a lot to go around
So finding it is really up to the individual and any potential contacts they may have. I would definitely try your Grandad's contacts as a leg up always helps to get hold of that allusive bit of land
Have a go at clays and see if you like it and how good you are. If you think it's worth training up a bit more and eventually getting yourself a shot gun of your own then you will have some land lined up hopefully, or at least in sight!
Also at clays you may well come across some shooting personalities who could point you in a good direction for some land to shoot over, or join a small syndicate.
Unfortunately finding a good bit of shooting is a bit like that of finding a top working cocker litter that still has a pup available, they're all gone before there's even been a chance for them to show up on practically anyone's radar!
I would join a shooting society (free or low cost) and quizz the experts on there, and also try your hand at a clay session and see how you go. If you love it you'll be hooked for life though!!
Good luck,
Best wishes
Mary