Thanks for all the replies and advice, I really appreciate it.

After lots of thinking about things over the last week, DH and I have come to the decision to put TTC on hold for another 6 months. I am not ready to say I am definatly not going to have another baby, but by waiting 6 months I will keep my next 12 months at work which will entitle me to my permanancy, something I have been trying to get for 7 years now, DD & DS will be just that little bit older- DD will be in full time kindy, we will have had a chance to get used to paying a mortgage etc. Also waiting that extra 6 months wont make a difference to which year the baby starts school, as the cut of in WA is June 30, so really it doesnt matter if I dont get pg for the next 11 months in that regard.
I feel much better for having made a decision, it feels a bi like a weight has been lifted off.