Which vehicles require a tachograph in the UK?

A tachograph is a device fitted to a vehicle that automatically records its speed and distance, together with the driver’s activity selected from a choice of modes. The drive mode is activated automatically when the vehicle is in motion, and modern tachograph heads usually default to the other work mode upon coming to rest. The rest and availability modes can be manually selected by the driver whilst stationary.
A tachograph system comprises a sender unit mounted to the vehicle gearbox, the tachograph head and a recording medium. Tachograph heads are of either analogue or digital types. All relevant vehicles manufactured since 1 May 2006 must be fitted with digital tachograph heads. The recording medium for analogue heads are wax coated paper discs, and for digital heads are digital driver cards containing a microchip with flash memory. Digital driver cards store data as a .ddd file that can be imported into tachograph analysis software.
Drivers are legally required to accurately record their activities, retain the records and produce them on demand to transport authorities who are charged with enforcing regulations governing drivers’ working hours.