Yes, the employee situation is difficult, but basically employer's rights are extensive. The fact that you're not employed as a photographer, or pictures were taken outside work hours, is irrelevant.
The basic question to ask yourself is, would I have been able to take this picture if it wasn't for my job? If the answer is no, as it often is, then there may be some doubt.
However, the intention of the law is primarily to protect the employer's commercial interests, not to stop you enojoying your legitimate hobby. That's the grey area.
Example: you work in the motor industry and get VIP tickets to the Formula One grand prix for you and your family. Very nice too, and pictures that you take out and about as a general punter, no problem. But if you get access to private areas and maybe get a shot of some secret new parts and then send them to Autosport magazine, you'll be in trouble.
Same time, same place, same circumstances exactly, but the content of the images, and their use, changes everything.
Edit: the definition of employee is another tricky one, but an easy check is if your employer deducts tax at source, you're an employee.