This is a common law offence which means that it does not come under particular legislation. The law surrounding it is from case law rather than legislation by parliament.
If you look at what must be proved that is basically what the law is in relation to this charge.
There is then a lot of case law that explains this law in greater detail.
1. Intentional and Unlawful total restraining of liberty of another person
2. Against his or her will
Lack of intent