If you can find them, American Classic Sprint 350s are a really good wheelset for the money, you should be able to get them for under £400 and they are under <1500gs. JEJames have them listed for £375.
I like my Zondas but had to search long and far for a spoke kit which was £30, there are three different spoke lengths in the wheelset so it seemed like a good insurance policy to buy one when I eventually found them. They roll really well though and I havent had any issues with flex.
Fulcrum 3s would be my recommendation if the AC Sprint 350s are getting hard to find. The Vision Trimax 30s get good reviews and GB cycles have them for 350 and they are worth a look.