The additional locum sessions and out-of-hours work is paid to you gross and you undertake it on a self-employed basis. When you first start self-employment, you are required to register with HM Revenue & Customs (HMRC) within three months or incur a £100 penalty.
If you are not registered I suggest you call the self-employment helpline on 0845 9154515 to register. Once registered, HMRC will issue you with a tax return to complete. To avoid another £100 penalty, you will need to file this online if you have not already sent in a return because the deadline for posting returns was 31 October 2009.
You must to declare all your earned income and any expenses claimed against your self-employed income for 2008/9 on the tax return. You need to file the return by 31 January 2010.
The tax on your self-employed income will be payable by the same date (31 January 2010) together with an estimated payment on account for the 2009/10 tax year. Usually, HMRC sends a payslip near the deadline to enable you to pay your tax.
As you are both employed and self-employed, you may pay more national insurance (NI) than you are required to. To avoid this, you can apply to defer class 4 NI contributions on your self-employed income. Download the application form at HMRC's website (www.hmrc.gov.uk).