Poppy - sorry just saw your message.
Yes you're right it will bring down your gross pay, but on what you earn/how much childcare vouchers you do/and how long you plan to take off, it may or may not be worth it. You have to do the sums I guess.
I work for myself now, so it's all irrelevant as you cannot get them until you become a limited company (which I am not).
Anyway, for me it was worth bringing down my gross salary by the amount taken off by childcare vouchers to have the benefit of having no adjustment to my months of only getting SMP, and a further 3 months off of getting no money at all, but still getting £243 a month in vouchers put in the account. So after 12 months off, I had £243 x 12, in my kiddivouchers account, it was almost 3k I think. However I had only lost out on 6 months worth of adjusted gross salary, which was about £200 a month off my salary (depends what tax bracket you're in I guess), but that was about £1200, so I was £1800 better off doing full vouchers. I actually put my amount up when I realised how much I'd make out of the deal, it's all based on your qualifying week, so you need to be giving that amount already, you cannot put it up after, only what you were sacrificing in your qualifying week will be honoured.
Hope that makes sense, I had the people from the daycare trust email me all the bumf about it, they were super helpful, as the company I worked for (a huge intl corporation) had no idea about it, and weren't too chuffed, however there was not a jot they could do as it's a nonworking benefit, same way as you cannot take somebodies company car off them during mat leave, because it's a non cash benefit.
Can send docs to you if you like, sorry if somebody else has already said this too.