Rex,
Is it at Lunar House?
If yes, there is only one entrance and the queue is about 1/4 mile at the best of times.
Take the train to Wimbledon, then the tram link (mention when buying ticket). Parking is expensive.
Exit tram at Whitgift Centre or just before. It's the tall building on the left.
On arrival, speak to a guard and see if you can get early entry.
0800hrs every day, the queue is half way down the street. I used to work opposite the place.