In reply to Will Collett:
You're looking for:
Free standing - so no seepage
Exposed to the sun and wind - dries faster
Little vegetation on top -less seepage
not in a forest - less humidity
Volcanic rather than something porous - so avoid St. Bees/Armathwaite sandstone
If you've got the guidebook handy it shouldn't be too hard to find something. The Bowder stone does take run off and I've found it wet before when expecting it to be dry but it might be worth a look. Eskdale will almost certainly be dry but would make the drive home longer.