If there is a redundancy, the company is obliged to go through a consultancy process. You have a right to know the details of the restructure including the new positions and if both of your positions are being redefined and amalgamated into one, then this should be made clear. You are entitled to speak to HR about this.
Read this and also consult ACAS.
www.acas.org.uk/redundancy
Regarding your current role and the changes that have been made to it - review your job description, look at the tasks that have been added and ask for a review to consider whether this additional tasks are reasonable, whether you have been given adequate training to complete them and whether they have been fairly distributed.
Whether these changes were brought in during your absence or while you were at work, is irrelevant, you would still need to be trained in them and given the opportunity to learn them, as would any existing or new employee.
Raise this with your boss and ask for a meeting with HR and advise them that you feel you are being discriminated against due to having taken maternity leave and that this falls under potential discrimination.
Make sure that your reviews are up to date, that all objectives are clear and that you have evidence to show that you are doing your job, meeting objectives and ask colleagues for 360 degree feedback.
Prepare evidence for your case that you should be the one to retain your job while also reminding your boss and HR that they cannot simply make you redundant because they want to create a new position to do the same job and to bring someone else in to do it.
If there are any female colleagues in other departments, seek out a more senior mentor, even if it's not in a related area and make this s formal part of your career development, or at a minimum, an informal situation where this person can support you and bring visibility to the process being applied in your department.