loisstella if you haven't been notified how do you know about the new role you will be returning to? Has someone been leaking information?
Yes they should be communicating with you if they haven't done so, absolutely. Not sure how you could use this for 'bargaining' though. What you are entitled to end up with is a suitable role, and it sounds as though you will. Yes they should follow an appropriate process but if the end result is as it should be you don't have much of anything to bargain with as you haven't ended up in a worse position because they didn't do it 'right'.
If you are on maternity leave, which I assume you are, they are not allowed to make you redundant unless there is no suitable alternative at all. So you are unlikely to get made redundant tbh.
Are they making redundancies generally? If you are returning to work after Additional Maternity Leave you are not entitled to come back into your old job necessarily if it's not practical to do so. This is because often in such a long time, things change in the workplace and your old job may not be available in it's old form. Is it that or is it a genuine redundancy situation where they are making people redundant, consulting with people, and just not involving you?
In terms of what a 'suitable alternative' is, it must be on no less favourable terms and conditions and be suitable and appropriate work for you to do. If the pay and other benefits are the same, but you have been made more junior, or the work is not suitable for someone of your seniority, you could claim that it's not a suitable alternative. So that is a possibility. But your argument would be that they must find you an alternative which is suitable, not that they should automatically then make you redundant. If they can't find you anything which is suitable, then you should be made redundant.
Is the work you were doing previously not now being done at all?
Tbh I wouldn't just sit there and wait to be communicated with. I would ring and ask for an appointment to meet with your boss and HR or whoever else is appropriate, to dicuss your return and establish what job you will be going back to.