I'm back!
Grace was bap 1793 in Farmborough and was Elizabeth's sister. Purnell is a surname of a non relatve on a completely separate bit of my tree, so I'll forget about that for a while.
Very interesting that John M was in Farmborough for this wedding as I had always fancied he never went there as he and Elizabeth didn't appear to have married in Farmborough (well, I had someone at the Somerset CRO look)
Thanks for finding that Kate