I could use 3 of these:
http://www.componentshop.co.uk/6v-9-0ah.html
That way if they draw too much current on 18v its easy to switch to 12v. But then I'd have to buy a set for each battle as there's not time for charging. How many battles does each robot have at an event on average?
The alternative I was thinking of was using a pair of 4s 30c 5000mah optipower lipos, hopefully I'd have time to charge them between battles. They would cost £90 for the pair.