KA92NR - Atlantic House is the place you're looking for.
The vehicle entrance is around the back of the main building, past warehouse buildings. *You may see articulated trucks around, it's near that area*
Once you find the gate just follow it to the control booth and say to the guy you're here for stage 1 assessment. You'll then go to the security booth, get a pass and then drive round to the car park.
If in doubt there are usually a lot of cars heading for the car park if you're going early morning. Easy enough just to follow them.