Trev
generous thickness of the plating may be giving you a false reading
Right, this is what happens with testing stone and acid.
The only reliable method is to file deep enough, but not too deep to damage the piece, and done where it cannot be seen (easily polished and buffed after testing).Clean the dust to make sure there are no remnants of silver and then apply very small drop of testing acid directly to filed area. The difference in color will be clearly seen.