Custom and practice
One of the ways in which a practice or benefit can constitute an implied term ( www.practicallaw.com/0-200-3351) of a contract of employment ( www.practicallaw.com/5-200-3117) . Such a practice or benefit will become an implied term of the employment contract if it is regularly adopted and is customary in a particular trade or locality or at a particular workplace. For the custom or practice to amount to an implied term of the employment contract, it must be "reasonable, certain and notorious" (Bond and another v CAV Ltd  IRLR 360) and followed "because there is a sense of legal obligation to do so" (Solectron Scotland Ltd v Roper  IRLR 40).
For further information, see Practice note, Implied terms in employment contracts: Terms implied by custom and practice.