The primary employer is BAA plc, find their website, then address, then where they advertise.
There are many secondayr employers, the ground handling agents. If you tour the terminals (or wait in this one!) then you will get the list of names. Track down the adverts for each company.
Some carriers do their own ground handling and, once again, you can track down where they advertise. In all, there could easily be ten companies to monitor and apply to. Depending upon what you want to do - there might be more.