I use Parasol IT.
Here's a brief run down of how the whole "pay" thing works.
You take a job and the agent sends Parasol a contract. Alternatively you can work direct to an employer without a agency (

) and use a generic contract.
Parasol then ask you to verify the contract via their website.
You then submit a timesheet signed by your employer to the agency and submit a similar online one to Parasol. This then generates an invoice that gets faxed/emailed to the agency.
The agency then pays Parsol who then do the tax and NI deductions and pay you. The agency then recoups it's money direct from the employer.
I submitted an online timesheet to Parasol and an physical time sheet to the agency last Wednesday for a job I was doing. The agency then ran its pay run on the Thursday.
The money was with Parasol on Tuesday this week. They then ran their pay run and the funds hit my account today. So I work approximately a week and a half in hand, so to speak.
You can also claim a daily subsistence rate and travel expenses through Parasol which you get as tax relief. You therefore pay less tax and this then covers some of your out goings.
Things to remember if contracting.
1.) Always have a cash float in the bank to cover at least 3 months bills etc.
2.) When between contracts, claim benefit. I claim JSA as a single man. Its doesn't pay much (about £59 per week) but you get your contributions paid and your legally entitled to it.
Yes, it is a bit of a hassle to sign on and then off, but I do it on principle.
3.) Always chase up your agent. Get a contract submitted ASAP. Always check to see that they get your timesheets. Always ask for things like confirmation emails. Don't rely on people doing their job efficiently.
4.) Be prepared to wait for your money sometimes (1 to 2 weeks) as some agencies are slow or have odd pay roll runs.
Hope this helps.
XoD.