Well yeah, basically you look at the 'employee' rate and double it, because as a freelancer you have to add in tax, NI, holiday pay, pension contribution, etc etc. you also have to consider business development - how many hours do you have to spend to get a day's work? That money has to br found somewhere. So your rate will seem high but it probably isn't.
That said, there is a massive variance in rates. It depends where you are, what the market will bear, and of course how good you are. The circumstances are also important - I do a big amount of work for one agency and give them a day rate of 250 - but I work for them practically every week, it's guaranteed income, and I have no business development costs for that income. Yes, I could squeeze my rate up, but I know they make a good margin on my rate, I'm easy to work with, everyones happy.
For direct clients, who are harder to get, I charge at least 400. But no-one else is making margin on that cost, IYSWIM.
Obviously I can't answer your question for you. But whether it's direct or through another agency has a massive bearing on your rate. Plus if it's regular work, etc etc. The minimum I would suggest is 200 pd and I only charged that when I first started out.
Oh and don't give a low rate thinking you can push it up later in the relationship - that never works.
Good luck!