Hi Guys,
good thread this, mine took about 3 years and 9 months including viva, 3 years and 6 to hand in, this is about average by the standards of the group i did it in, 2 members in my time pretty much handed theirs in after exactly 3 years and viva'd 2 months after that, so it can be done. Probably easier to do if it's in the sciences and all your experiments work pretty much from the outset, which for most of us they rarely do.
BTW 4Matt, I'd check the regulations on your 1+3 PhD, I think you get a year's grace to write up, but don't get paid for doing so, thus enticing you to try and get it in earlier I suppose. Normally if you take up all your writing up year as well, the institution often starts to charge a "registration fee", or thats how it works where I am.