Ah... apparently it is in the county archives - this hasn't come up in any of the searches I've done over the past couple of weeks and I found it about 2 minutes after posting this question! They do claim to have copyright:
http://www.chesterimagebank.com/terms.html
I wonder if the webmaster I originally contacted didn't reply because he'd never asked permission to use it either?!
I will ask the archives and hope they don't want to charge a fee. It's an interesting issue - I found the same with the Wellcome Trust image library, which charges for commercial use of pics - even ones that were widely available as cheap prints in the 18th/19thC. I don't know how they could prove that I haven't found my own copy in the attic or something...
Anyway, thanks for the replies!
C