On 5 October 2008, amendments to the Sex Discrimination Act 1975 came into force which now means that women on maternity leave have a statutory right to all the terms and conditions of employment that would have applied had they been at work, including all non-cash benefits except for their remuneration (or normal salary). Instead of her normal salary, a woman on maternity leave is only legally entitled to receive her maternity pay, either statutory maternity pay (SMP) or in line with an enhanced maternity pay scheme such as under Agenda for Change. Non-cash benefits are not considered as remuneration and therefore have to be paid in addition to maternity pay.
This change to the law means that employers now have to continue providing all non-cash contractual benefits for the full 52 weeks of statutory maternity leave regardless of the period that a women receives maternity pay (either SMP or enhanced maternity pay). Recent guidance from HM Revenue and Customs indicates that childcare vouchers are non-cash contractual benefits which should be continued during all statutory maternity leave. Those women who sacrifice their salary to receive childcare vouchers will still be entitled to receive them even when they are only receiving SMP, as SMP cannot be reduced to cover the cost of childcare vouchers, or if they are in nil pay.
For the HMRC Guidance see here