You are looking for this door on your right when you are in the car park.
Map and directions below
Leave the A419 as before, don't turn left into Liden Drive but continue round the roundabout and take the third on your left into Faraday Road.
Go to the bottom of Faraday Road to the roundabout and turn right into the car park and the entrance shown above will be on your right.
Press 707 and enter on the door entry system on the left of the door and I will open the door.