Wow, thank you for all of the replies and apologies for not coming back sooner! There is loads of stuff to consider here, and you've all given me lots of helpful insights - especially those from people who are currently in the teaching profession. It's really valuable to hear about your experiences, so thanks for taking the time to respond.
In answer to your question, changeisagoodthing, no I'm not catholic. However, I have been known to exhibit workaholic tendencies, so I take your warning on board!
Stoat - yes, would definitely want to do GTP at a school like my daughter's! To be honest, I think it's the sheer brilliance of dd's current teacher which is inspiring me to think about this whole thing again - she has a lot to answer for! What she does is incredible, and really worth getting up for every morning!! :)
Interesting that you think the age could be a positive thing, tryingtobemarypoppins2 - I hadn't thought of it that way, I must admit. I would be looking at FT work, unfortunately, as I couldn't afford to do any less! You're probably right that now isn't the best time to be getting into teaching, given all the changes that are going to be brought in. In any case, I'd have to save up for a couple of years in order to make it viable!
Pigglesworth, you're right that I should talk to DH. I guess I'm just trying to figure out how serious I am about this whole thing before I scare the living daylights out of him! 
Thanks for the many reality checks re salary etc. I'm nowhere near London, and I think I have a realistic idea of what sort of salary I'd be looking at after a few years. Definitely a lot less than what I'm used to, but we're not horribly over-committed on our mortgage and we've reduced it considerably while interest rates have been low through making big overpayments. So we could probably adjust...
Good to get the reality checks about the politics, the bureaucracy, the stress etc. I'm not afraid of hard work at all, and particularly not for a job that I think is worth doing. However, the potential difficulty in finding a job is a major factor that would concern me. A bit concerned too about the relative worth of the different routes into teaching - there are clearly some different views as to whether the GTP route is as good as the PGCE route etc.
I'm still quite confused, all in all, but this has given me plenty of ideas to ponder. Quite a few people have suggested staying in my current job and doing something more rewarding in my free time (what free time??!?), and I think I really need to consider this option and figure out if there might be a way of making it happen. Stressheaderic, it's really interesting that you find your youth group work more rewarding than your teaching career.
To those of you who are making similar gambles of this nature, good luck! I'll come back and update when I've figured out what to do!!