These kind of long-term contracts pay the 'market rate'. Back around 2000 the market rate was about 30-40GBP/hr or about 300GBP/day, or higher in the city. Although I haven't done any contracting in the UK for quite a while, I do occasionally look at the contracts out there and the rates seem to be about the same these days. Just go to jobserve.com and you'll see.

Right now I live in Canada and charge US$100/hr for short term contracts (up to a few days), although normally I quote fixed-price and it works out roughly $100/hr or a bit higher. This is mostly for customizing my own product to a customer's requirements.