Worker status depends on an individual being obliged to carry out services personally for the other party to a contract. If a worker has a genuine right of substitution, they are not under such an obligation. Stuart Delivery Ltd v Augustine is an example of a case where the option to release a shift that an individual had previously agreed to work was not an unfettered right of substitution, meaning that the courier was a worker for the duration of each shift.
Mr Augustine worked as a motorcycle courier. He was offered "slots" by Stuart Delivery (Stuart). If he signed up for a slot (which he was not obliged to do) he agreed to be available for work in a particular area for a particular period in return for a guaranteed minimum payment. Once a courier had signed up for a slot, they could release it, which would make it available for other couriers signed up with Stuart to accept. However, if no-one else accepted the slot, the original courier remained liable to complete it and faced sanctions if they did not. Stuart argued that the ability to release slots in this way meant that couriers were not under an obligation to carry out services personally and could not be workers.
The tribunal and EAT disagreed. The tribunal's primary finding of fact was that Mr Augustine did not have a right of substitution at all. He could hope that if he released a slot, someone else in the courier pool would relieve him of his obligation to work. But he had no control over whether anyone, or if so who, would accept the slot and remained obliged to work it himself if no one else accepted it. In the circumstances the tribunal was correct to find that there was no right of substitution. It was also correct to find that Mr Augustine was not running a business on his own account, so he was a worker for the purposes of the Working Time and National Minimum Wage Regulations, amongst other things.