I was tuped to another company during my employment. The new company then relocated and I was made redundant. My original employer has stopped me doing temporary work at the same building stating to the agency that ex employees are not allowed in the building. I could understand this if I had been sacked but I was not.

Unless there are some security issues, I don't think this is legal. On the other hand, if the former company owns the building, they can refuse entrance.

But the bottom line is the agency is going to listen to the company, especially if they are a client.