Should I get paid double for working bank holidays?

Often an employer will pay staff a higher wage for working on bank holidays, but this shouldn't be confused with a legal requirement, and there is no formula saying how much more you should be paid for working any given hours.

Any extra payment is up to your employer to offer, and to state in your contract when you agree to take the job.

However if your employer has regularly offered this in the past, such that it has become 'custom and practice' to offer, and they are now refusing, it could count as a breach of contract. Consult your union rep if you have one or seek individual legal advice if you think this might be the case.