Ladies bikes

Newbie post.

Great first post.

I live on a boat [...] Thoughts are a mixte frame - I'm 6' near as damn it
There must be something about tall women living on boats:

http://www.lfgss.com/thread74433.html

:)

is 21" big enough
As Ed said, getting advice in a good bike shop is a good idea. It's impossible to judge just from the frame size, i.e. you also need to know your inseam measurement, and there are other factors to consider. It's not rocket science and at the end of the day all it boils down to is that the points of contact you have with the bike are spaced in a way to suit you. You clearly seem to want a fairly upright riding style, which further influences your choice.

Finally, I know that 56a Infoshop etc do bike maintenance but the advice I've had there before has been pretty hit and miss; are there bike co-op type places where someone can show me how to lace a couple of spokes, leave me to do the rest and then tell me what I did wrong when I'm finished?
Try the Hackney Bike Workshop--it may be far away from you if as it sounds you live in South London, but it's well worth going. If you have any specific questions or want to do something specific like wheelbuilding, you can PM Zebra Cyclist on here beforehand and he can tell you what to bring:

http://www.lfgss.com/thread77137.html

There are three every month, at two different venues, so don't worry if you miss this one, although it will obviously be the last one this year.

It's extremely busy but there are a lot of very knowledgeable volunteers on hand.

Have fun!