&my role is being eroded by my maternity cover*
What exactly does this mean?
Will your role continue to exist? Who will be performing your job going forwards?
Tbh... this is dodgy ground for your employer and they could be at risk of sexual discrimination.
We would never make a woman on maternity leave redundant, as it's very risky and there would have to be a lot of justification for it.
It sounds to me, that they don't want you back and this is their opportunity.
The way they said they'd look at a deal, doesn't indicate that it is a true redundancy situation.
If I was in your position...firstly I'd think about whether I wanted to return anyway.... if I did, then I suggest you seek legal advice from an employment lawyer, who would examine your letter and seek clarification.
If I did want to leave, then I'd be looking for a years salary from them.
Did they notify of this in writing? If not I would ask them to.