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

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