Rob,
We get ours from Build Centre (nice and close) we pay £3.30 for a 25kg bag (special rate) which covers roughly 25 m2. Make sure the sand is not damp as it will have to dry out on the paving before you can brush it in.
Also it should take you about 1 hour to brush in 100m2.
Hope this helps 
Regards
Alan
From home 09:00, drive to build centre, pick up sand from yard, go to counter, wait in line, go to coffee machine, chat with Brian, drive to client, start sanding, drive to Goodies for breakfast,chat with dolly at counter

, drive back to client, finish sanding, no sand left on surface all joints filled to bottom of bevel, drive home 13:00, make a sandwich, if its a Friday - walk to pub 14:00 - 17:00, Stagger back from pub just in time for dinner

as I said don't underestimate how long sanding takes :

Regards
Cliff