There are very few employment cases to refer to as illustrations. But it has been decided that the custom of paying non-unionists a lower wage than unionists was unreasonable, and that even though an individual employee was unaware at the time he was hired of the custom of making deductions for bad workmanship in the Lancashire weaving trade, the custom was nonetheless "notorious" (i.e. well-known in the locality or particular industry).
If the parties have included an express or implied term in the contract, then it is not permissible to prove a custom inconsistent with the term.