Hiya Spendlove,
I am wondering whether Mabel may have been a nurse at the front during WWI. If she did not marry, which is possible (maybe the love of her life was killed in the War), obviously you would not find a marriage. The other thought that occurs to me concerns her father. Was Rev. John Bird moved to another parish or diocese? Would Mabel's death be recorded anywhere other than Bath?
Best Wishes, Chris