The test which has been devised to identify true contract workers is based around whether the worker is under the supervision, direction and/or control of the client. 

Anyone who can evidence to their employer that they are outside of SDC can claim expenses incurred as a result of fulfilling the requirements of their contract.  These can be reimbursed either through salary or direct from HMRC at the end of your contract or tax year.

HMRC definitions of SDC are as follows:

Supervision is where someone is overseeing another person doing the work according to a required method.

Direction involves advising another person to do their work in a certain way, generally by providing instructions, guidance and advice as to how the work is to be done. This can be via a procedure manual.

Control is where you have someone dictating how they should go about doing that role.