hi cedcommerce, thanks for your reply.
I have gone from tearing my hair out to kicking myself – I can’t believe I missed that. DOH!
Yes, that worked just fine.
As it happens, the site in the subfolder started off as a duplicate of the site in the root of the domain. When I installed that, I duplicated and altered the db – I really should have remembered to alter the siteurl and home values.
Thanks again for your help.