It’s all very silly isn’t it? Babs isn’t old and doddery. She knew that Sally was selling. It would not have been hard for her to go to Sally and say “thanks for letting me use the land, can I buy it from you so that it doesn’t get included in the sale of your house?” Sally might have transferred for a notional amount, or Babs just paying the conveyancing fee or something.
Clearly Sally didn’t see it as significant to the value of her house and she was right because OP didn’t either.
Instead Babs stuck her fingers in her ears and closed her eyes then got all arsey when the new owners reasonably wanted access to the land they had bought. I wonder if Babs has a partner who was equally as slow on the uptake? I wonder if the neighbouring “allotment” owners knew Babs didn’t own it, and what they thought? You’d think one of them would have suggested to Babs that she get it sorted before the sale. Maybe they all hate her though?