Hi, I've dne the Plain a couple of times though nothing recently. Suggest you get hold of the wiltshire rights of way officer as he/she will be able to advise you what is and isn't open. Rumour has it that some of the lanes have been closed by the military and not just while manouvers are taking place.
Word of warning on the plain, there is a lot of ordnance out there that hasn't gone bang, so don't stray off the tracks (and don't touch).
Remember, keep the greasy side down!