oh god yes I hear you there. That is bascially exactly the situation we were in when ds was born. We'd cleverly just moved and mortgaged ourself to the hilt just before he was born .
There are ways round this. Eg you could flexi school (have your daughter in part time). You could work evenings etc. Terramum makes some good suggestions.
Most HErs are skint, tbh. Most of us are living on one income in a two-income society, but plus also-HEing does, for my family at least, have certain expenses that I don't think are so vital when you have with kids in school. Am thinking the endless arts and crafts stuff, books, games, educational trips, etc. YES I expect you have these when you have kids in school, and YES they can be bought pretty cheaply BUT-I think we just need more of them, really. Theres little things like providing more meals (assuming you'd get free school meals at school), more heating, etc. When you are near the bone these things can add up.
OTOH it does mean that there is a money saving mentality in the HEing community. There are normally lots of resource sharing opportunities, if you fancy the the big get togethers, they tend to involve camping or youth hostelling, etc etc.
So I'd be willing to bet that it CAN be done for you! BUT BUT BUT I think the real problem here is that your dp isn't really on board. And am not sure what to say here really, why do you think its a good idea and he doesn't? Would he consider just keeping your dd out for one more year and reassessing in september 09?