How many weeks are you, hattyyellow? Can you ask for another scan now, just to clarify the situation?
Basically, the lower the placenta is, the less likely it is to 'move'. (It doesn't really move, btw, but it's just that as the uterus expands it takes the placenta with it, like a sticking plaster stuck on a balloon which you gradually inflate.)
If you do have to have a c-section, it will be a lovely calm elective section, probably done quite early though (36 or 37 weeks - no later). I know it's not what you want, but it needn't be a disaster, honestly. I had it, and although it took me a while to get my head round the fact that I wasn't going to have the lovely aromatherapy and whale music home birth that I wanted (OK, not aromatherapy and whale music really... just the home birth bit ), I can honestly say that dd's birth was amazing and wonderful and 8.5 years on I remember every moment of it as one of the two best days of my life (the other was when dd2 was born).
If your placenta is covering the os, I'd avoid sex and heavy lifting, etc. There's no need for bed-rest, though, just be sensible. I'd also have a bag packed just in case you start bleeding and need to go into hospital at short notice, and also some help with your other dcs on standby. All this may not happen, but it's good to be prepared, just in case. I also wouldn't go anywhere without my maternity notes!
Good luck. I really hope all works out well for you.