Try Radial Cycles. I think they have (among other tyres) continue Gators at £20 each or GP4000s for £25 each...I don't think you can beat those prices...they may have other makes if you prefer something other than Conti.
I used them late last year, not the fastest delivery but at the price, unbeatable.
I'd never heard of them, still don't, might be worth doing some homework on their performance.