I've got a bit more time now, so if you want, you can you explain a bit more about how/when Sibelius is crashing. Obviously I can't fix BBCSO or Sibelius, but I might be able to help narrow down the error. Screenshots would be useful. (I'm offering because I'm helpful rather than because I think anything I did is causing the problem).
If you can look at the Documents/Sibelius Services/Logs folder and send the install log to chris@sibelius-services.com I can also check if there's some clues.