Ok - I think this time I fixed it for good! I actually purchased an "FG radio amplifier / booster box" from FTG Autosalvage, but whilst testing with this new box, and my one at the C-pillar, I relaised what the issue was.... The box is earthed by the bolt thta holds the it mounted to the C-pillar, but this point actually seems to provide an extremely poor and intermittent earth connection! The solution - see picture attached, I got an alternat earth which seemed much better from an adjacent bolt. Now the reception is flawless! Another issue that Ford warranty was unable to fix, and I had to fix at my own expense and in my own time.... Jake.