Most of the Handling Agents in the UK now only recruit temporary staff through employment agencies at the start of the summer season.
Find out which agencies the HA's at STN use and apply to them, although as it's July you may have to wait until next year as most HA's should have their summer staff by now.
If you hear of a new airline starting up/moving routes to STN then you might find one of the HA's recruiting out of season so keep trying!
The pay and hours are awful, but it's the best job on the airport!