I think your idea about fitting a heater, oil radiators are great, and a nice warm bed in your shed is a great idea, if someone could pop round in the morning and evening to check food, water and cats health, I would also leave a letter for your vet or contact your vet directly asking if ok that if your cat needs any attention you will pick up bill on your return. Aslong as your cat flap has a two way lock it would be easy to contain him/her evenings then open two ways again mornings.
