You should be getting at least the National Minimum Wage for every hour spent on your night shift. It makes no difference that you are asleep or that nobody ever wakes you up during the night. You are still working. You should be paid the National Minimum Wage because your job is to be at the service user's house throughout the shift and you could be disciplined by your employer for leaving without agreement.
It's not as if you can pop out when you feel like it, to go to the cinema. If you are not sure about the legal position, speak to your union rep. If you are not yet a union member, browse our Union Finder tool to find the union best suited to your needs.