Answering my own question some hours later, the trim panel in front of the seats does not easily pull off, also being secured by plastic trim at each side. But I was able to ease the bottom of the panel away just enough to squeeze my fingers underneath and retrieve the lost phone - now also partially wedged under the rear edge of the carpet. Safety warning: this is not a painless procedure!
Lesson for the future is to organise rolls of fabric or similar to fill the gaps at the outboard sides of the 3rd row seats.
