In reply to Bhealey:
A lot of peak gritstone dries off quickly. You want to look for a venue that is facing SW and in the SW/W wind. Even at this time of year wind and sun dry the edges very quickly. If there is not wind and sun after rain at the moment then forget it!
Southern Sandstone, including harrisons, needs a good dry spell first so you dont break the rock, recently wet sandstone can snap without warning - it become much more fragile./