My top tip for temporary workers.

If you take a temporary job you will likely be on an 'emergency' tax code. This is identified on the payslip as 'BR' on the part where the tax code is printed.

My advice is to contact the tax office after your first payslip and make sure they issue your correct tax code to your employer / agency at the earliest possible opportunity. Follow this up with your employer / agency as soon as possible to ensure this has been received and is being used. You should receive a tax refund via your next payslip after the correct code is issued.

Under the old system (JSA) any overpaid tax was refunded to you via DWP / ex-employer (if they still held your P45 as some agencies do) or HMRC (via a cheque). This money was yours to keep and did not affect benefit payments.

Under UC, things have changed and any tax refund will be treated as 'income' by DWP. If you receive the tax refund during your new claim for UC (after your temporary job has ended) then it will be included in your UC calculation and your UC will be reduced that month. The tax refund will not benefit you if that happens.  

Keep an eye on your payslip as you never know what you could be missing

You can also do this online:

https://www.gov.uk/log-in-register-hmrc-online-services