Hi Ron, this reminds me of something I saw on that BBC series about the Planners, maybe it's on iplayer still. Anyway there was a chap doing up big old trucks, unimogs etc as a hobby, big boys toys. He was eventually granted PP for a large shed to work in. There was no hint of commercial activity, yet the objecting neighbours were using the argument that it was commercial activity and therefore not suitable in a residential street.
I'd ask the local planning office how these activities are considered under planning laws etc. There may be some issue with the activities going on so long before hand unmentioned, has this ever been reported before? If so an enforcement officer might be aware, and hopefully give some weight to your case - assuming it is a change of use, and they haven't got permission to be doing anything now, let alone applying to increase the amount of industrial activity going on.
Photos, diaries, video footage etc may all help, as would something from Environmental Heath on the noise - there are restrictions to what noise anyone can make at certain times of day and night.
Good luck
