Brilliant Avia - I think you've cracked it. Yes her father was Arthur Frederick, sorry that was a mistake, it was her brother who was Arthur Frank John. So it does look as she was a relative and perhaps arranged the marriage between Mabel and a neighbour Thomas Harris, knowing that both were struggling to bring up children on their own. It was still a big gamble wasn't it, to go all that way and marry within hours of meeting. They must have hit it off, as a daughter was born a year later!
Thank you very much, that's another puzzle solved.