A quick update:
I had a chat with the guy (Charlie) who runs the Facebook page, and I am related to the Craggs which is exciting. I discovered that my great grandfather, James Henry, died of tabes dorsalis (late stage neurosyphilis) in 1930.
I still have no idea what happened to Matilda. She disappeared between the 1911 and 1921 censuses and I've not been able to find a death certificate under Cragg or Williams, but it's just this minute occurred to me that she might have taken the name Craig too, so I'll look for her with that name. I feel really sorry for her; James either had syphilis when they married (in which case he hid it from her), or he caught it after they were married so obviously had at least one affair. I'm assuming that it was him, not her, that brought it into the family, but Charlie seemed to think that was the case.
It also makes it even more likely that grandpa changed his name to dissociate himself from the family - he probably felt that both Williams and Cragg were somewhat tarnished.
James lived with his brother John Albert in 1921 and was probably already showing symptoms of TD. John's wife was also Matilda's sister and Charlie thinks that another brother married another Martin sister.
