In a nutshell, I completed the form with my income details on for the years asked (2016/2017).
I had seen a lot of people mentioning that you get more tax credits if you're on Maternity Allowance because they don't regard that as income (only first £100 of SMP is income), and if your new tax Years income is more than £2500 less, you should notify them.
So what do I do? I did call the tax credits line before sending off the form and they said to complete it as usual, wait a few weeks and then give them a call to inform them I'm receiving Maternity Allowance so they can adjust my claim