I'm looking at buying a composter for my allotment, from my local
council.
As far as efficiency of composting, which is better - one 600 litre
composter (£15) or two 330 litre composters (£6.00 each)?

composters work best when they are filled in one go and the contents turned
when it has cooled down. If you have enough waste to fill the big one, then
go for that. If not, I'de recommend the two smaller ones.