Well it largely depends on how different the job is in terms of job content and terms and conditions as to whether your original role is redundant. It's not a question of how many changes an employer can make, it's an overall picture.
Even if your old role is redundant, that doesn't mean you are entitled to redundancy if the new role would be considered a "suitable alternative", which means same or no less favourable terms and conditions, and job content which is suitable for you in terms of your skills and experience.
How much further away is the office location?
A change in manager isn't really relevant.
Perfectly possible to have job description alterations without the post being redundant.
If you feel it is a completely new job, what is happening to the job you were doing?
Also try turning it on its head, and ask yourself the following: Imagine you didn't want redundancy and wanted to stay in your job, if they made your role redundant but wouldn't offer you or let you apply for this new/altered job, would you think that was perfectly fair?
If you are on maternity leave you also have some additional protection in that you are entitled to be offered suitable alternative work where it exists over and above other candidates. Its not surprising they are avoiding any question of your redundancy.