Whilst on maternity leave an employee remains precisely that, an employee. Treating them different to this will likely lead to a discrimination claim under the Equality Act 2010.
Throughout maternity an employers obligations to the employee remain the same, as do the employees to the employer.
Examples include continuity of accrual of annual leave and length of service and the right for the employer to expect employee confidentiality.
The only contractual term that is not mandatory and clearly set out is that of remuneration.