WAKING NIGHTS

On a Waking Night our carers are on call at all times during the night and are awake, alert and making certain our client is sleeping or resting or at least safe, sound and secure. Our shifts are from 9pm/10pm through to 7am. We also offer flexible shift partners, which suit our clients.
Can a client offer us a sleep in rather than pay for the times we wake up to a client.
Is there a law that protects us from not being paid for waking nights?
What are the rules around this?
When addressing this question, it is logical to see what the law says about the differences surrounding sleep -ins and waking nights.