Most contracts are at large enterprise size companies, like isp's, banking, government. Those companies don't do direct business with contractors but post their contracts through brokers, here some large ones:
> For Central London, a developer should be asking for £300-£600/pd depending on seniority and what they're doing.
That seems pretty low to me. Here in the Netherlands contract developers get about the same, between £400-£600/pd (actually between 70~90 euro/hour) but cost of living is way lower than in central London.
https://qualogyportal.secure.force.com http://www.it-staffing.nl/ https://www.headfirst.nl https://www.freelance.nl/