Advise Please

[willyup]

There's all sorts of ways they could have this covered in your contract.
Some employers will forbid you to get involved in any other business no matter what it is and others dont mind as long as there is no conflict of interest.
The more senior you are in an organization the more strenuous the restrictions are likely to be. In some cases it wont matter as long as you have declared the outside interests to your employer.

As said above, get legal advice. It's the only way you'll know for sure where you stand.