Sounds like a great event, i'd love to be involved. Plus i only live 20mins away so i can bring terrain if needed
EDIT: Alternatives to the format outlined about:
1) three doubles games (always fun)
2) one huge day-long battle across the three tables (i.e. two 'teams')
2) A smaller battle (doubles perhaps) followed by a larger battle (two teams)
Well, i have 3 friends with warmaster armies, not asked them yet. Is there a maximum number of players? .. i think that would take us up to 10 players, if that's too many it's okay.
I do like the idea of one huge battle across 3 tables, it's not very often i get the chance to have a game involving that many people. Can you imagine a 5-a-side warmaster game, each with 2k points

But i don't mind which we do, i'm up for the smaller games as well. Word of warning, if we go for the massive game, i'd try and avoid the tables in a long line (18ft x 4ft) because from experience it ends up feeling like it's 3 seperate games anyway. For massive games you're best to do either a T-shape, or maybe 4 tables in a square (12ft x 8ft) and special deployment (so you've got armies all over the battlefield)
The other thing to be aware of is the time. I've done a few huge warhammer games in the past and in all the games our problem was we ran out of time. When we turned up we were chatting, laughing, fetching drinks, looking at each others armies, discussing our tactics. All the things we should be doing because it's a sociable event. I think we finished 4 turns, it's amazing how fast the time goes. It would be a real shame if we organised all this and didn't have time to finish one game.
Not sure what the solution to this is...... if you aim for a game to last 3 hours and there's 6 turns, that means 15mins per side.