Thanks for the replies - pretty much as I thought, neither myself nor the guy who sits on my left with all the answers could think of a reason for switching off a GPS.
As far as reception goes, my mate had to hold it up to the cabin window to get any signal at all, which is probably what attracted the CC in the first place.
I can understand the principle of "if in doubt ask for it to be switched off", but I can't recall anything from balpa regarding GPS used by pax. The only stipulation in our pax safety brief is "the use of electronic devices during take off and landing", but that's more because we don't want laptops flying around the cabin when I smack it a bit too hard onto the runway.