Dizzymamma, what a dilemma.....It's doubly hard when someone has recommended you, as it's such a great feeling, knowing you are good at what you do, and wanting to get more involved.
I am in a similarish situation. I was in a SMT job before having ds, then went back 3 days a week (keeping my management responsibilities, but part of a jobshare). It was incredibly hard work, even only doing 3 days. I felt I was doing 3 jobs badly - being a mother, being a class teacher and being the SMT bod who is supposed to support others. We then had to move, and I decided not to work for a bit, and now, a year later, I'm looking at jobs again. I think if I wanted to work fulltime then it would be relatively easy to find another SMT job, but I really don't as it took up so much of my time (8am - 6pm most days, then planning at weekends etc) and though it was hugely rewarding it was also exhausting and stressful.
Are you in a job-share at the moment, and would the school consider a jobshare application for one of the SMT posts? It can work really well, as the school usually gets more for their money. That could be a possibility.
If you really really want the job then do go for it, but be realistic about how much of your time it would take up. I'm not sure the "try it for a bit and see" approach is advisable in teaching, because as you say, the expectation is that you'll stay for at least a year. I would also say that if you have the qualities and enthusiasm for a more senior post (which you obviously do as your Head has already earmarked you ), then those abilities aren't going anywhere - there will be other jobs coming up over the next couple of years, probably within your current school, and certainly in other schools. It's not a win/loose situation in that if you don't apply, then you'll never get the chance again.
Having said all that though, I desperately miss my job, and the buzz from children and working hard, and can perfectly understand what a difficult decision it is, so not much help really.