Daz joyce

hi all would anyone be able to give me some advice, as i am going crackers were do i start, a while ago, the we went for a meal, the car was great, we came out, it was completely dead, nothing , ie nothing, went to the battery , wiggled the battery cable, everything back on, and started, got home, fine for a week, then completely dead again, did the same thing, all came back on, took the car to jaguar, and they said it was the negative lead playing up, so had it replaced and car was all good for a month, yesterday wife calls me, saying both our fobs have failed and she cant get in the car, i replaced both batteries ,still nothing, used emergency key, car completely dead again, call RAC. they put there jump leads on car, and it started to drain his battery, so he disconnected it, on his computer the battery said 1 volt, this is a brand new 900ah uassa , he thought it was the alternator that had gone, and done battery, we were both just looking in the boot, when all of a sudden the car locked its self, he put this jump leads back on, and the car started instantly . we gave it 10 minutes, he put his test equipment back on, tested the alternator perfect, it had started to recharge my battery, we checked for battery drain when turning the car off, 0.02 ,so nothing, so he had done his job and got car started, i turned it off, left it for 2 hours .went back out and it started instantly. so from a new battery with 1 volt showing to a battery showing 14.76 volts and a starting car within 3 hours. so i have no battery drain, i have a good battery, i have a good alternator, the fobs are working now, and i have done nothing, what is going on, as an electrician its doing my head in, i have noticed there is a small connector plugged into the negative lead, its this the bms , if so, if there is a intermittent break in the cable, could this drain the battery to nothing, and when contact is remade work again, no fault lights on dash, no fault codes any advice on my problem