Hi Bryony,

I've been there, done that. This is what was suggested to me by the adoption group I adopted the 2 young cats from...
Put the kitten in a room with a closed door. The older cats can smell, but can't see or contact the kitten. Do this for about a week, or until the older cats seem to be comfortable with the situation. After a few days you can introduce a "toy" so they can interact with each other. I used a sturdy piece of string, and attached a spool to each end, since the spools did not fit under the door. Put this under the closed door, so that there is a spool on each side of the door. The cats can now play with the kitten but still not harm it.
After a week I put my new young cats (6 and 9 months old) in an enclosed ex-pen (this is a play-pen for puppies, sort of ) and let my cats and the dogs get used to the new additions for a few hours. Then I opened the ex-pen and let the cats loose, under close supervision! Everyone got along well. A lot of sniffing, but no hissing etc. It's been 5 years, and they all still get along!
It's worth a try at least. Good luck!!!!!!