As mentioned earlier in this thread confirmation has come back from Alain at XP Metal Detectors stating "you can update the coils even if they are on a metallic surface", no problem at all
So it does appear the earlier information was purely just a rumour.
You only have to be careful before you switch the machine on for use that it is away from anything metal so at least 15 inch`s is recommended.
Cheers all
mrix