That's not exactly the case, Lumps... Child care vouchers are seen as a benefit and therefore they have to keep paying them throughout your maternity leave - on top of normal maternity payments. Like how you still accrue holiday and you still receive company wide bonuses.
So as long as you use the vouchers (they last for 12 months, so can be saved up for when you go back to work) then you're actually saving money on tax reduction. Though it will reduce your immediate cash in hand in your salary.
Arguably you could stop them between 17-25 weeks and then start them up again, to receive maximum salary and maximum vouchers, but I think most employers would take a dim view of blatantly playing the system that way!
In my case, I think there's a clause in my contract preventing me from changing my vouchers more than once per year, anyway.
For those who are working for a company that does childcare vouchers and who intend to use them when/if they go back to work, it's actually worth starting the child care vouchers before you go on maternity leave if you're on statutory maternity pay.
My negotiation with work will be over KIT days. As they can technically subtract the childcare voucher amount from any money owed for a KIT day - which means my going in would be utterly pointless as I'd get nothing for it that I wasn't getting already!
Hoping I can negotiate that they pay me on top of the childcare vouchers. [mercenary]