Finally making headway!
Well, I have finally been put in touch with a company based at Robin Hood Airport, or Doncaster if you prefer like I do. They generally deal with the big metal but I'm guessing it's all the same principals. I tried the smart phone application but I don't know how much I trust it, as I can get a reading and move around but when I get back to the same point the reading can be different. I did try it in the aircraft though and the cockpit seemed to pick up the same amount of magnetism as outside, whereas in the engine bay it went ballistic! But I don't know enough as to what is magnetic that shouldn't be etc, and if I could have picked a reading up from the mags