Its going to vary based upon the number of people with similar skills, the complexity of the role, etc..
There are a few websites that seem to specialize in contract work..
www.jobserve.com seems to be popular... also
www.jobsite.co.uk is another.. There was a posting on here a while back with a list of good websites for employment, you can probably find it if you search for it..
If you're new to the UK, it can also be harder to get a high paid role because they seem to value UK experience above equivalent experience elsewhere.. but contracting can also be a good way to build local job history quickly..
Good luck!