Hi, I might be able to help as I'm a HR manager.
It really depends on your employers policy. My employer allows back to back maternity and rather than basing Pay calculations on actual average earnings (like SMP does) they base it on contractural salary. (This is a new thing though in our company and won't necessarily be the case everywhere)... If your company only pays SMP then it should, in theory, still be based on your average earnings during the qualifying time. (I think its between week 17-25 of your preg)
The best advice... is to contact your HR and ask them, if you don't agree with them or it doesn't sound correct then contact CAB or ACAS to check... Happy for you to PM me too if you have any other questions... x