If an employee does not return from maternity leave, does the employer have to pay her in lieu of the annual leave that she accrued during maternity leave?

Yes. An employee on maternity leave is entitled to all her contractual rights (except remuneration) so annual leave continues to accrue during the maternity leave period. If the employee resigns at the end of her maternity leave period the employer must pay her for her accrued untaken annual leave. If the employee resigns with the termination date effective before the end of her maternity leave she will be entitled to payment only for leave accrued to the date of termination.