"I fully understood the terms, not a registered kitten, price £300.00 cash (she gave me a vets card showing it's date of birth and date of full inoculations without request from me).

After I gave her the £300.00, she asked to take it to a vet in a few weeks and have it neutered, I agreed verbally to this"

This is a vital difference. It comes AFTER you made the main contract. Although you orally agreed that, there is no "consideration" for that as a contract to amend the main contract. You have no obligation to neuter the kitten.