beccas, it sounds like they are saying, not that they won't honour your enhanced Mat Pay, but rather that they won't be replacing you while your gone?
That is perfectly acceptable. They don't have to fill your vacant post while you are gone, and they can break your current role down into elements and spread it about to avoid the cost of paying another staff member.
If your current job involves delegation (it sounds like it), then it probably isn't unreasonable to ask you to look at how your jobs can be delegated.
Also, they can't rely on you taking the full 52 weeks off. The law says they have to presume you will be, but that you are allowed to return prior to 52 weeks if you give 8 weeks' notice of your intention to return. So you can tell them now that you intend to take off 52 weeks, but they can't act on it.
Additionally, they only have an obligation to give you your exact job if you take up to 6 months leave (OML) The next 3 months, which are still paid with SMP (so weeks 27-39) and the last 3 months which are unpaid (so weeks 40-52) of your Mat Leave are Additional Maternity Leave (AML). If you choose to take AML, you lose your right to the exact same post as you left. Instead you have a right to either your original job, or a job that is similar on no worse terms or conditions, if it is not possible to give you your original post. You have no automatic right to vary your original post in terms of hours, although you can put in a flexible working request.
Is your concern that they will realise that with efficient spread they don't actually need a person to act as 'office manager'? In that situation you would be in a redundancy situation, yes.