Not terribly secret given the large signs as you drive from the Bristol direction - and from the air it stands out like... anything. But I heard they were very keen to discourage passers-by driving up to the gate in the picture
You can see the guard tower in the top picture above just to the right of the woods - I presume they have sensors on the road as well. Someone told me it was used as a Cruise Missile deployment location out of Greenham but they are still all top secret other than "Salisbury Plain" where a close friend ran into a deployment one night in the '80's - he claimed he was on a rally but we all suspect he was out dogging.