Permitted work

If you are getting employment and support allowance (ESA) , incapacity benefit (IB), severe disablement allowance (SDA), national insurance credits or income support (IS) because of incapacity for work you are allowed to do some permitted work.
You do not need the permission of a doctor to do permitted work but you should tell the Department for Work and Pensions (DWP) if you are working. 

Have a look at this page of the Disability Alliance website, and free to download fact sheet (updated 31/1/11) for up-to-date information:

http://www.disabilityalliance.org/work.htm