In reply to GarethSL:
It's unpredictable. It could be covered in snow, it could be tshirt weather, but you'll only know when it's closer to the time and you can check a short range forecast.
Tbh April, as someone else stated, is really the perfect time for grit. The rock isn't greasy, and the midgies haven't started yet.
Also be prepared to go to another crag just in case it's blowing a gale.
If you're here for a week you'll definitely get some dry rock (but realistically probably a fair bit more).
Have a good trip.