I just typed out a really long post with all of the backstory and issues and emotions involved, and Mumsnet bloody are it before I could post it! 
This is the short version - we love our tiny Victorian terrace, which is in a fabulous, central location and a friendly community, but the DC have only tiny bedrooms, which is going to become more of an issue as they get older (they're 9 and 7); and it's just generally too small, and we're in a bit of a black hole as far as secondary schools are concerned. Our best bet would be the two local single sex schools - we have a DD and a DS - but they're not guaranteed and we're not sure we want them at single sex schools.
Moving out to the suburbs would get us a bigger house, more space, and would put us closer to the excellent co-Ed schools. But we'd have to run a second car (and pay for extortionate station parking), DH's commute would get even longer, and instead of living in the bustling, pretty city centre, which we love, we'd be on a fairly soulless estate with nothing much around us.
Over Easter we decided to bite the billet, and have appointed an Estate Agent, but I'm having massive second thoughts. I love our little house, I'm more worried at the thought it might sell than it might not!
We can't extend it any further than we already have, and as well as the kids' tiny bedrooms there's no study or quiet area for DH and he's going to be WFH more often in the future, so that may become an issue.
It feels like a head over heart issue, and I really don't know what to do. Please advise, wise Mumsnetters!