If your agency gives you the option of using an umbrella company, RENEGOTIATE YOUR HOURLY / DAILY RATE before you accept. This is because the agency wants you to use an umbrella company because it can and will stop paying your employer's national insurance contributions, and you'll have to pay them yourself via the umbrella company. If you don't renegotiate, the agency will simply keep that amount. These agencies are very greedy. They charge the school I work for £170 per day and pay me £105. Out of the huge £65 profit they make every day for nothing more than a couple of phone calls, they are supposed to pay my employer's national insurance, but they've wriggled out of it.