I have been involved in summat similar from the otherside. The difference was that the taxman was the one arguing not the employee or the employer. The rules do seem to have changed but the taxman says that they haven't. He says that the rules weren't properly enforced before. The argument came down to this. If you work thirteen weeks for the same employer, if you do not control when you work, if you do not arrange your own work, if you do not supply materials or if you are not a limited company then you are employed.
In the end the employer had to find the national insurance and tax for fifteen people, this backdated to the company's financial year. They were lucky to have negotiated this since some of the chaps had been on the strength for years.
This is, as far as I can see, a result of building industry abuses.