Some info here, but if you're in the UK or somewhere cold, you'll have to shelter it over winter in very cold weather so the pot doesn't freeze

http://homeguides.sfgate.com/make-wisteria-bonsai-71031.html

That's a very good website, Bamboo, not least because it is truthful about the size of the flowers!

Englishrose - I grow many bonsai, and they reward me each year when the leaves grow successively smaller. You'll need your Wisteria seedlings to grow to 2' or 3' to carry any blooms, as the flowers do not grow smaller (i.e. they're out of proportion to the leaves). But good luck, and do consider sharing some photos.

They must look a bit weird when in flower then...

They do!