OK to answer my own question, the start button works once you are in the car, because....
.....this is a TEMPORARY solution. This is due to the fact that, when you use the fob, even once, by pressing unlock to open the door whatever, it turns it back to keyless entry. Glad I found that out before ditching my farraday pouch !
So if you disable it on the fob using the battery saving method as Thackeray described, it will only be disabled until you press a button on the fob.