Contract rates are always higher than normal wages. Best way is to work via a Ltd company.
If you really do mean contract basis, you're liable for NI, Tax etc. and will need to employ the services of an accountant. The upside is that you can expense a hell of a lot of stuff to save tax (ie, breakfast, lunch and dinner if you're at work for a long day). But still, I'd expect contract rates to be at least double those of the salary.
On the other hand, if they've offered you a 3 month contract on the payroll, to do the job and see if you're any good at it, then they're just being cautious / taking the mickey, depending on your perspective.