Am I entitled to a bonus and, if so, is it entirely discretionary?

Whether you are entitled to bonus payments depends on whether there is an agreement between you and your employer.

If there is a contractual right to a bonus, i.e. there is an express term or implied term, (say by custom and practice), in the contract of employment, then failure to pay the bonus, where due, would be a breach of contract.

If it is clear that the employer has discretion over whether to pay a bonus, it would appear there is no contractual entitlement. Even so, it has been held that, even where the employer has stated bonus payments are discretionary, because the parties intended the bonus would be paid in all but extraordinary circumstances, a bonus payment was due.