Thanks everyone for the helpful suggestions.. I haven't researched mooring yet, so will get started on that.
I like the look of this boat that Alan de Enfield suggested:
https://narrowboats.apolloduck.co.uk/boat/rd-fabrications-55-widebeam/415746
Mainly because of the size and the fact that it looks like a project that I could comfortably work on.
But I'm going to start looking at Broads Cruisers and Narrowboats too. I did live on a narrowboat in Oxford in the early 2000's and I seem to remember it was around 45ft. Was very comfy however I'm concerned about space... I should really spend some more time aboard each type if I can to see what would actually work. I know with many things, they seem small on the outside but once inside there's lots of space.
Thanks Jen, I'm going to have a look at the Broads Cruiser info that you sent too.
Alastair