Nah, sorry, this isn't 'the seller was wobbly of mind and totally forgot the coffee tin containing 5K she had stuffed under the floorboards in 1983, and therefore did not actually intend to leave the buyer a 5K gift'...
The seller sold the property including the allotment plot. She will have gone through the details of the deeds and what the property consisted of with her solicitor and with the estate agents prior to sale. The particulars of what she was selling will have been discussed several times during the process.
She could possibly have sold the plot seperately, she could have gifted it, she did neither thing at any point in her ownership of it, but had she genuinely wanted to sell or gift it to Bolshy Babs, surely she would have done so.
It is possible its not permitted for the plot to be separated from the property - that would be in the deeds somewhere, @YaMuvva could possibly confirm that.