There are two issues here, and the first and foremost is your employment status. It sounds like you are an employee, regardless of how your post is funded. If you have been continuously employed since 2007, then you are deemed to be on a permanent contract, even if your current role (not contract) is on a fixed-term basis. That simply means that when that role comes to an end, if there is no other role to redeploy you to, then you would be dismissed by reason of redundancy, not for the end of on fixed term contract.
Your employer should have given you any changes to your terms & conditions in writing, which they clearly haven’t done since March 2010, but that doesn’t alter the fact that you are on a permanent contract. So you should be entitled to any terms & conditions that are commensurate with that – which brings us to the second issue, that of the maternity provisions.
If the enhanced provisions are contractual, then I can see no reason why you shouldn’t still be entitled to that. If it is discretionary, that still doesn’t mean your employer can arbitrarily decide who to give it to or not. If it has been this way for a considerable time and you would reasonably have expected to receive it (as it would appear) then it’s more than likely that it would be classed as custom and practice.
I suggest that you raise the issue of your employment status with them, pointing out that you are now permanent and they should confirm that to you in writing, then raise the issue of the maternity pay. If they refuse to do either of these things, ask them to objectively justify why not – I doubt they will be able to do this, and you will probably have a strong case for breach of contract.