"Worker" means an individual who has entered into or works under (or, where the employment has ceased, worked under) (a) a contract of employment; or (b) any other contract, whether express or implied and (if it is express) whether oral or in writing, whereby the individual undertakes to do or perform personally any work or services for another party to the contract whose status is not by virtue of the contract that of a client or customer of any profession or business undertaking carried on by the individual; and any reference to a worker's contract shall be construed accordingly"
"The significance of mutuality is that it determines whether there is a contract in existence at all. The significance of control is that it determines whether, if there is a contract in place, it can properly be classified as a contract of service, rather than some other kind of contract."