Thanks for all the replies. It all sounds a bit scary - but in an exciting sort of way

From the sound of it though, I don't think I can manage this on my own, so I've rung a teacher locally who specialises in arranging to ask whether he could teach me at my level. He said he'd normally expect somebody to be Grade 5 on their instrument, but he was in the middle of band rehearsals at the time so he's going to ring me back to talk about it. He mostly does it by hand, but uses Sibelius if he wants to hear what it sounds like. I asked him about the clarinet not going above the G on the top line with Sibelius but he didn't seem to think that this was the case. Perhaps it's just with the Lite version that that happens? It seems like a major flaw if that's the case.
I could be 3 years or more away from Grade 5

It's not as if I want to arrange orchestral music, it's only relatively simple song work so hopefully he might think he can teach me.
On some of the points people have mentioned, it would be great to get a MIDI system some day. The other day I played along with somebody who has a MIDI system - he'd pre-recorded the piano parts so that he could play with me on clarinet and it sounded great, with all sorts of special effects

But like everything else, there must be a huge learning curve to get to the stage where you can do that.
I take on board the comments about what is technically possible rather than theoretically possible, and also that a notation program is a tool for the initiated rather than a magic wand. I need a teacher....
Sarah, thanks for the offer about the Finale piece. I'm tempted to take you up on it, but if I get a teacher who uses Sibelius, then I'll no doubt get Sibelius as well because I'm sure I'll need as much help with the program as with the actual arranging
Petrat, composition interests me and I'd like to learn more about it. I've written some music for a couple of songs but I stress that they're very, very simplistic and just for fun. So yes I'd like to do what you suggest, but I can't take on more exam studying just at the moment, much as I'd like to

I think it's more likely to work the other way with me - if possible. As in, I learn how to do simple arrangements first, which gives me a head start when I come to do composition.... Not sure if that will work, it may be the wrong way round
Thanks again everyone, and I'll report back on what the arranging teacher says