I know Hellaby. Easily would seat 1,000.
This is a pure guess and speculation. Perhaps the circuit that attended is 20 congs x 80 publishers so equals about 1600 pubishers. So too big for one assembly so they divide it in two. So in theory 800 per session.
Apathy could easily drop this into the 600's.
Any football matches in Sheffield that weekend to distract? Oh yes - Sheffield Wednesday vs Barnsley in the Championship league - local match and all that.