I've checked in Lasors, and as usual, it's a bit "hazy":
No person may operate an aircraft radio station in the air, or on the ground, unless they are in possession of
a FRTOL, or are operating directly under the
supervision of the holder of a FRTOL. The latter implies
that the FRTOL holder is present in the aircraft.
Authorised RTF Examiners at Regional Test Centres
It's the final part about "implies" that gets me! I'm just wondering how many PPL's are out there inadvertently contravening this!