The law states that 'every person driving or riding ... on a motor bicycle when on a road shall wear protective headgear.'
The regulations apply to both the rider and the pillion passenger (i.e. in the seat behind the main seat) of two-wheeled motorcycles, whether it has a sidecar attached or not. However, they don't apply to a passenger riding in a side car. If any 2 wheels are less than 460mm apart, they're treated as one wheel. Some motor mowers are exempted from the regulations. Helmets must be worn and properly fastened in order to avoid committing an offence.
A follower of the Sikh religion who wears a turban is exempt from the helmet requirement.
If a pillion passenger aged under 16 doesn't wear a helmet, both the driver and the passenger will be liable. Otherwise, drivers and riders are responsible for wearing a helmet.