You need to agree a rate which is 20% at minimum higher than you expect to be paid to factor in Employers NI, Umbrella Fee and Apprentice Levy.
In my field this took a few years to factor in but rates have increased to cover these additional costs (almost).
At one time Agencies offloading payroll benefited both the Agency (lower payroll costs) and the Contractors (HMRC allowable expenses) however as HMRC has now pretty much stopped all such allowances the Agency wins as low payroll costs and we absorb all additional costs.
The idea from HMRC is we as contractors must be treated as employees (hence no expenses anymore) and we as employees would receive similar benefits from employers .. not in my world has that happened! We provide a fluid and dynmanic workforce that helps businesses yet we'll never realise the benefits of being employees!
/rant