Welcome Ash Kate 1
Have you hired before at all? How many cabins will you need? There's so many options that whilst I would tend to book with the boatyard direct, you might find the Hoseasons website useful to get an idea of the different kinds of boats available and the prices. Not all yards go through Hoseasons either but I think that you would find members here who would probably recommend all of them for various reasons of personal preference.
https://www.hoseasons.co.uk/boat-holidays
In my experience Richardson's are always very competitive on price and hard to beat for that. However they are a very large boatyard and some people prefer the more personal touch of the smaller yards such as Bridgecraft and Summercraft. Norfolk Broads Direct have a good variety of boats, always well turned out. And personally I love hiring from Ferry Marina as Horning is a good starting point.
A lot may depend on your basic requirements and the budget you have in mind. There are a number of yards offering discounts for late bookings if you can hold your nerve.