Hmmm...my version of ffmpeg seemed a little odd - so I rebuilt latest stable from source. Now it tells me:
ffmpeg version 2.0.1 Copyright (c) 2000-2013 the FFmpeg developers
built on Aug 26 2013 15:24:53 with gcc 4.7 (Ubuntu/Linaro 4.7.2-2ubuntu1)
configuration: --enable-libfaac --enable-libx264 --enable-libmp3lame --enable-pthreads --enable-gpl --enable-nonfree
libavutil 52. 38.100 / 52. 38.100
libavcodec 55. 18.102 / 55. 18.102
libavformat 55. 12.100 / 55. 12.100
libavdevice 55. 3.100 / 55. 3.100
libavfilter 3. 79.101 / 3. 79.101
libswscale 2. 3.100 / 2. 3.100
libswresample 0. 17.102 / 0. 17.102
libpostproc 52. 3.100 / 52. 3.100
Which does not satisfy the libavcodec <=55
I then went for a a slightly older version (1.12 - which seems to be a fork of the main project, but still linked to from ffmpegs main download page)
That gave me:
libavutil 52. 18.100 / 52. 18.100
libavcodec 54. 92.100 / 54. 92.100
libavformat 54. 63.104 / 54. 63.104
libavdevice 54. 3.103 / 54. 3.103
libavfilter 3. 42.103 / 3. 42.103
libswscale 2. 2.100 / 2. 2.100
libswresample 0. 17.102 / 0. 17.102
libpostproc 52. 2.100 / 52. 2.100
Which passed the configure check for tvheadend, and gave me the missing checkbox
My attempts so far to actually get transcoded output have failed...but I'll try again later :-)