Sylvie was found today & is now home safe & sound! She had somehow found her way to a church some distance from where I live - one of the congregation saw a poster today & called me.
If I may, I would give the following advice to anyone whose cat is currently missing:
1. Don't give up
2. Follow all the advice listed on this website: http://www.catchat.org/discus/messages/7/223261.html?1281119481
3. Posters. Put posters everywhere you can & even further afield than you think your cat might have gone - I would never have thought that my cat would have ended up as far afield as the church that she did (she's neutered & has never been prone to roaming before)
4. (Worth repeating) Don't give up
Thank you all for your help, advice & kind words.