Katieerocket - I think often at 6, children don't always have one particular friend. DS has one now but its only literally been in the last few weeks that the same boy keeps being mentioned. Before this he would change his friends more often than he changed his socks! Think Sparkles has a good idea in relation to activities for your DS. This may also help to get your face known in the area too.
Sparkles - its such a shame your friend is acting like this because from the sounds of it you had a really good friendship as did your DD's and your DP's. Once you move maybe you should make a lot of effort to visit her, invite her to your house etc etc. If she is feelig insecure this should reassure her that you have not disappeared. If however she is still acting like this after you have made the effort you may have to resign yourself to the fact that it is actually jealousy and not insecurity. I do hope its the former though.
Sanae - its an interesting point about idealising your old life, and it doesn't help for me when I go back to visit and just have fun when I am there without the everyday stresses and strains of juggling work, kids, housework etc. Perhaps the tried and tested pros and cons list may help? Why may it be taken out of your hands soon?
Tutter - am living in the Dearne Valley now. Where are you originally from? Where are you now?
MammyofET - yes - having a supportnetwork does make a huge difference doesn't it?
Picalilli - where in Sheffield are you? Are you still in touch with any old school friends or is it a possibility that you could call some of them and arrange a catch up meet?