It's held mainly indoors for the radio stuff in a sort of big modern 'shed' they call the hangar, with a selection of outdoor stalls selling the usual military surplus stuff, clothing, equipment etc.. Lots of interesting things to rifle through, domestic radios, components, microphones and speakers, racks of fascinating MOD expensive-looking equipment, old walkie talkies and cb paraphernalia, rolls of cable, that sort of thing. Last time I went there was an outdoor stall with literally buckets full of bulbs, croc clips, jack plugs, indicators etc.
Hack Green itself is an interesting place to visit, so it's a full day out by the time you've been over the stalls and then had a wander around the nuclear bunker and gazed at the rotating radar which they usually have going for these events.
You pay once for the fair and then again for the bunker, or whichever you choose. Free parking right next to the place. Tea & cakes available in the hangar and then a full-blown restaurant in the bunker.