I discovered this issue because PMS is running on a Seagate Personal Cloud that is too low powered for any transcoding. The BBC iPlayer Channel requests transcoding even when though this is not necessary as I am using clients (Amazon Fire TV & Roku 3) that are capable so are set to play Original Quality.
The error from the Fire TV is "The server is not powerful enough to convert video" whereas on the Roku 3 the error is "Conversion failed. The transcoder crashed or failed to start up."
The BBC iPlayer native applications on each client work flawlessly. Interestingly the Live TV streams do work on the Roku 3 Plex Channel. Hitting the * (Option) button gives me this information:-
Quality 720p (Original)
Direct Play Force
Audio Unknown (AAC Stereo)
Allow Direct Stream (ticked)
Vide H264
Playback Direct Play
Format hls
For those programmes that are not live if I hit the option button after the player appears but before the transcode failed error message the information displayed is identical apart from:-
Format mkv