The standard personal allowance (amount of tax-free income) for April 2013/14 will be £9,440 which allows an individual to earn £181.54 per week (about £787 per month) tax free.
This equates to 29 hours worked in any one week before our temporary workers start paying any tax (based on National Minimum Wage)
Interesting fact: a temporary worker based on National minimum Wage can work for 3 x 8 hour shifts, from April 2013, tax & NI FREE.
From April 2013 the class 1 primary NI earnings threshold for employees will be £149 per week (£7755 per year).
Full details of Income Tax and National Insurance rates and allowances from April 2013 are available on the HMRC web site