In reply to gribble:
They will be biased. They will make more out of you as a Ltd. Company.
You have three choices. Sole trader, umbrella, or Ltd. Depending upon how you get your work will limit these. Agencies do not like sole traders alas. As a sole trader you would be paye, self assessment, and pay type 2 ni (nominal). However the agent can be liable should you not pay you tax properly. Generally they won't deal with sole traders.
Umbrella is basically like being a permie. You'll pay tax and two sets of NI. Employees and employers. However it's simpler and you will not need an accountant
Ltd is potentially the most tax efficient. You will have more stuff to do though. Like being competent in running your our business, know about tax etc. Though as others have said one man Ltd or psc's may be subject to change soonish. Or not. If you have multiple clients this may not be an issue. You will also need personal indemnity insurance etc.
If you go Ltd. Check out crunch accounting. They will be cheaper than sj but you will need to do some stuff yourself.
Being self employed is one of the best things I have ever done.
The best of luck.
Cheers
Mark