Thanks so much for your reply.
Yes, Mary may have been illegitimate, we know nothing about her parents. She'd still have her birth registered though wouldn't she?
Her eldest son Harold was named after Mary's husband John's eldest brother. The second son they named Joseph. Her daughters were Gladys then Lena. I have messaged Lena's daughter to see if she knows anything about where the names came from, that's an excellent suggestion - thank you.
The marriage witnesses were friends of John.
All my mum remembers being told about Mary is that John 'brought her back from a farm in Wales', she was illiterate and very poor. Seeing as John also lived in Wales and was also poor she took that to mean that she came from very rural Wales and absolute poverty.