In reply to Kim:
Thanks, that sounds likely, I'd never heard of bans on the edge and there was no sign of birds.
Regarding the sign itself. Going to the crag we crossed the fence that contours the slope a few hundred feet below the crag via a stile which is at the very bottom left hand corner of the the quarried area and there was no sign there. When we left we crossed it via the next stile left (looking up hill) about a hundred yards towards the water tunnel and this was where the sign was.
Nobody would sensibly access the quarry via this stile (perhaps nobody would sensibly access the quarry by any stile, but that's another matter ). Perhaps there was a sign on the other stile originally but it's now missing?
The date definitely said until 31st August and I'm almost certain it didn't specifically refer to the quarry, I should have taken a photo but I didn't think at the time.
Anyway, looks like another trip to the edge is permissible, thanks for clearing this up.