Jane - There won't be any banjos for sale at Sewerby, we've never had any dealers there in the past. You will, however be able to get lots of advice and hear and try out a lot of banjos - should be maybe 10 helpful players there. You'll get a much better idea of what's good and what it's worth
I agree about music shops - they don't usually know squat about banjos, but try and convince you they're experts. Most banjos in music shops are so badly set up they're pretty well unplayable. However, in Yorkshire there are some shops that DO know what they're talking about. Here are some:

Eagle Music in Huddersfield is the main Deering dealer in the UK, and sell Goodtime banjos for below £300 - these look very basic but play and sound great. They're also very light (less than 2 kilos). They have a banjo day this Saturday (all day), with Greg and Janet Deering from the USA, workshops in all banjo styles, and a bluegrass concert with the Kruger Brothers. It's not free, however. See their website.

The Music Room in Cleckheaton is a good place, that often has some second hand older banjos with the more modern stock. They also deal in most folk instruments (fiddles, bodhrans, boxes etc).

Mannings Musicals in Bradford is a dealer in used instruments that often has a few good five string banjos.

I've never been, but I believe Hobgoblin in Leeds is good too.

I give lessons in playing fiddle tunes using clawhammer, but I'm in Doncaster.
Best of luck with it - no matter what most Mudcatters say, we need more banjo players.
Cheers
Ray