Wedding registrar paid, but never attended, cancelled shortly before wedding

That's shocking! Did you have to cancel the wedding or did you go ahead with the ceremony and have the legal bit done later? Either way, you shouldn't be left out of pocket because of this. And that's before we get into any stress resulting from not being actually able to get legally married on your chosen wedding day.

I would start off by contacting the registrar. Beyond that, write to the Registrar General at the General Register Office as they are the people who 'oversee' all English registry offices.

That sounds very unusual and not something I have ever come across before. Were they a council registrar or were they a celebrant.