Last time we used Bridgecraft at Acle. You only seem to be able to book them through Hoseasons now.
http://www.hoseasons.co.uk/v3/Webpages/EUBoat/ProductDetailPage.aspx?ISDLNK=1&SCODE=BRID&ACODE=BH1545&TIDPDEUBOAT=boatInfoVery pleased with the boat. The good thing about this yard is that there's a pub next door and the river is quiet here, very useful if you haven't been before and need to get used to the boat. I wouldn't recommend one of the bigger yards up near Wroxham for a first time although the prices are cheaper (Richardson's for example). Also, if you can afford it, get a bigger boat than you need. The first time we had a tiny boat and the experience was not altogether a pleasant one.
We haven't been for years though because I tend to get bored half way through the week and want to go home!