Bookworm
I can't see this being a job that will take the bill much past £250. This is if the unit was instaled IAW the TSO in the first place.
From the instalation of my unit it has had NO limitations at all but we paid close attention to the details of the TSO and installd the GPS in the way that the manufactures intended. I see a lot of instalations that have had corners cut in the way that they have been put into the aircraft.
The biggest problem with GPS is the technical side of the CAA had it's act together long before the operations people and so some reprisenatives of the CAA imposed limitations on instalations that they should not have or were only approprate to that opperator of the aircraft when working under public transport (AOC).
The whole thing is being blown out of proportion after all if you fitted a new ILS in the aircraft would you be going through all this nonsence?..... I think not!!