Dan,
My son, father-in-law and brother-in-law are all published composers and all use Sibelius extensively, so I can probably get answers to specific questions about it.
I see you are London based, so if you are willing to travel to Epsom (30min from Waterloo or Victoria) you can even have a play with Sibelius (with an attached MIDI keyboard) if that helps. My son is a weekly boarder, he will be home tomorrow (Sunday) but away at about 6.00pm.
PM me quickly if you want to take this up.
Oh, and another thing, Sibelius tech support are usually very good.
W