I had to frog the whole thing!
Well, live and learn. I tried it on after turning the heel and picking-up stitches for the gusset, and it was too small! Shucks.
I've moved on to a different pattern, this time toe-up. I did actually learn a lot with the cuff-down pattern, but I'd prefer to knit a pattern which can really be tried on as it goes.
So, now I'm working on this pattern - http://secure.elann.com/ShowFreePattern.asp?Id=5024
and I am having the hardest time with the toe. I understand how it goes, but I can't make it work. The stitches are very tight and getting in there to knit is incredibly difficult. I'm off to try casting-on again (5th try! lol). If anyone has advice on this toe technique it'd be greatly appreciated.