After an accident you have been involved in you must stop your vehicle and, if required to do so by any person having reasonable grounds for so requiring, give your name and address, the name and address of the owner of the vehicle and the identification marks of the vehicle (Section 170(2) of the Road Traffic act 1988).
The duty to stop means to stop sufficiently long enough to exchange the particulars above: (Lee -v- Knapp 1966 3 All ER 961).
Section 170(3) places an obligation on the driver, if he does not give his name and address under subsection (2) above, to report the accident to a police constable or police station as soon as reasonably practicable and in any case within 24 hours. The duty to report means "as soon as reasonably practicable": (Bulman -v-Bennett 1974 RTR).
It does not mean the driver has 24 hours within which to report the collision.