This is step 7 of our guide Contracting in the UK.
Rate are driven entirely by market demand and are either hourly or daily.
Whilst an initial rate would have been agreed the agent they will often try to negotiate further with you to get themselves a higher margin. This is standard sales and you will need to learn how to negotiate your rate to ensure you get the maximum you can.
As mentioned before, make sure you determine your own market rate before entering the market.
With a little practice you should ensure that you only ever pay an acceptable agency margin and maximise your income.
After you have negotiated your rate you will need to arrange a payment structure to ensure you can get paid.