Hi Kirsty,
You can do it yourself with mkvmerge part of the MKVToolNix package (v5.8.0 now out) as long as you have the subtitles or can get the subtitles you need. With mkvmerge you can add or remove subtitles, change the order or force them. It then remuxes the stream so its usually done in 1-2 mins

Quick answer
1.open mkvmerge
2.Drag and drop the mkv into the input window
3.In the track window below will appear the Video (V_MPEG), audio (A_DTS), subtitle/multiple subs (S_TEXT)(if you have them) and Chapters (if you have them)
4. Highlight the S_TEXT line
5. In the General track window options confirm the Language is correct if not set it. Change the Default track flag to yes and the Forced track flag to yes.
6. Click start muxing and it will out put the file your-movie-title (1).mkv. This will have forced subtitles.


If i get more time at the weekend i'll add some more detail, and show adding removing and changing subtitle order. For those looking to modify subtitles then Subtitle Edit I found the best e.g. time stretching, auto spelling correction, auto fix common error etc