A: The PAYE system is set up to collect tax and other payroll deductions. When an employer is due a refund that sum is first off-set against the PAYE due to be paid over every month or quarter. Rather than pay back the balance of the refund to the employer, HMRC hang on to it to set it against future tax/ NIC liabilities.
If you look in your business tax account under the PAYE section and click on payments made, it should show an unallocated credit, which will be the balance of the SMP owing. You can ask HMRC to pay that sum to you, but that will involve phoning the employer helpline.