When I applied it was a stipulation that you MUST visit an operational unit prior to your second stage interview, and the contact details for certain units were sent out to you by HR. You can contact HR directly on the freephone number available on the NATS website, or try looking in the AIP Aerodrome section (available on the AIS website) - the aerodrome entry for Stansted should have the tower number on it. Good luck!
MR