I'm not in work mode, out of touch / out of the loop I don't have childcare so can't attend meetings etc or work for more than an hour at a time. I have a small baby and I'm very tired trying to fit in writing this in evenings My line manager is off sick and can't help
How did they 'break the news' to you that your role was being downgraded. Did you get a proper letter explaining their rationale/justification for why your role lost 12.5% value ie did they say it was based on industry or role benchmarking?
I can imagine it feels a real downer being remote from your workplace and not being able to talk with your manager
"I just really resent having to work on this whilst I'm trying to enjoy baby"
That's understandable but if it's company-wide there's probably not going to be a lot that could have been done about that. Your question was whether you have any cause for complaining, and the answer depends on things like how well they've communicated with you and involved you, given you enough time etc.
My understanding is that for 9 months you can go back to exactly the same job you left
No, that's wrong. If you take up to 26 weeks (6 months) maternity leave you have the right to return to your original job. Once you go beyond that (as the OP has done) the employer can offer you a similar job if it is not possible to give the employee their old job back.