There are specific rules relating to Sunday work (and betting shop work)
HERE but these make no mention of any rights to receive extra pay. The law is concerned with the right of an employee not to be dismissed for refusing to work on Sundays. That being the case, this is, as stated above a matter of contractual obligation. Even if not written into her contract, it has become customary that your mother receives pay at double time for Sunday working, and as such the employer must give reasoning and adequate notice for the variation of contract. Your mother is entitled to accept the terms or reject them, but ultimately if she rejects them then the employer is equally entitled to dismiss her and offer her employment under the new contract. She
may then have cause to take action for a breach of contract or unfair dismissal but only if the correct procedures are not carried out and there was no good reason on the employer's part for making changes or dismissing her for refusing to accept the new arrangement.