FVP 4000/5000 Media Player Subtitles

The media player on the FVP5000 has very basic features and seems poorly supported by Humax. The feature I have found impossible to use is the use of subtitles on video media. played from a USB stick

The media player apparently has the option to turn on subtitles, but I have not been able to get to see any. I have some non-english language films and I can get an external subtitle for them (an *.srt file), but these are not supported by the media player. To get around this have tried encoding the subtitles into the movie file using VLC. This can produce a movie file that plays with subtitles on all of the video players I have tried on my laptop, plus will play happily on my XBOX ONE media player - but still not on the Humax FVP5000. I am guessing that I just need to find the correct format for the subtitles in media files to get it to work, but I don't know what that is. Anyone got any idea?