Oh dear, it's one of those'it all depends' cases. The basic rule is that the courts are against restricting someone's right to work. But, much depends on what the work that they have been doing is. If they are a hairdresser, it might just be reasonable to stop them working for a salon down the road because there is the risk that they will have a list of clients who they might entice to following them. If they are obliged to work a few miles away, that is unlikely to happen since the clients would be reluctant to travel. So the first question is, what does the term in your contract actually say and the second one is what is the reason for it? It needs to be a very good reason for it to be justified. Prima facie a motor factor has nothing in the business that would require such a condition or one set to such a distance.
The cases that get cited about such restrictive terms in general, are usually of little help. They are generally where the employee has done something verging on the criminal, by taking what should be trade secrets, vital to the business, with him, and blatantly using that to the advantage of his new employer, and, of course, himself. That is highly unlikely to be the prospect in your case.