Thanks both for your replies. I knew about the 1901 census already, thanks GeoffE, I was wondering if it had changed hands (maybe to her parents! but unfortunately not!)
Do you think if a single woman was living at that address, she would have worked there, or do you think she could have lived in a flat above the shop or something? Her marriage certificate didn't show an occupation.
I would have thought it unlikely that, if she simply worked in the butcher's shop, she would have given that as her address on her marriage certificate. More likely that she lived at the same address, in a flat over the shop. I presume that you have ruled out any family connection with the Mr Mawer, the butcher?
Go to www.francisfrith.com/search/england/lincolnshire/louth/photos and you will find several old photos of Mercer Row in the 1950s/1960s. The shops all appear to have had accommodation above them, usually two storeys. So I think this is the answer to your puzzle - namely that she happened to live in accommodation above No 30.