For example, it may become customary (over a period of time) to leave early on a Friday, or to add an extra day to a Bank Holiday. In order for an entitlement to become established by custom and practice, it must be long-standing, uninterrupted, automatically received, expected and well-known.
It is not enough to show that something has happened for a particular length of time. Instead, an outside observer, looking at the behaviour of the employees and the employer, must be able to conclude that both parties must have intended their practice to be a contractual right or obligation, with legal force. This is very difficult to establish.