Hello Dan,
I've attached an MKV (With DVB Subtitles) to this thread seeing as no one else has.
I'm a bit surprised that you don't have tvheadend setup already somewhere seeing as you're in the video editing business, I'd assume you'd want to test out your program on as many sources as possible.
Anyway hope this helps.
MediaInfo
Format : Matroska
Format version : Version 2
File size : 20.3 MiB
Movie name : Channel 4 HD
Writing application : Tvheadend 3.9.2513~g1d8b541
Writing library : Tvheadend Matroska muxer
Video
ID : 1
Format : AVC
Format/Info : Advanced Video Codec
Format profile : High@L4.0
Format settings, CABAC : Yes
Format settings, ReFrames : 4 frames
Codec ID : V_MPEG4/ISO/AVC
Width : 1 920 pixels
Height : 1 080 pixels
Display aspect ratio : 16:9
Frame rate mode : Variable
Original frame rate : 25.000 fps
Standard : Component
Color space : YUV
Chroma subsampling : 4:2:0
Bit depth : 8 bits
Scan type : Progressive
Language : English
Default : Yes
Forced : No
Color primaries : BT.709
Transfer characteristics : BT.709
Matrix coefficients : BT.709
Audio
ID : 2
Format : AAC
Format/Info : Advanced Audio Codec
Format profile : LC
Codec ID : A_AAC
Channel(s) : 6 channels
Channel positions : Front: L C R, Side: L R, LFE
Sampling rate : 48.0 KHz
Compression mode : Lossy
Delay relative to video : -2s 43ms
Language : English
Default : Yes
Forced : No
Text
ID : 3
Format : S_DVBSUB
Codec ID : S_DVBSUB
Language : English
Default : Yes
Forced : No