I've swapped roles with the woman who used to work under me and she covered my leave.

It's CRAP. She did my old job on 28hrs a week, when I had 40, now she's full-time she hasn't taken back the responsibilities so I'm doing her old 20hr-a-week job AND what took me about 15-20hrs a week too all on just 20hrs a week. She should get her finger out big-time! But how do I tell my manager when I've been doing it for months now?

Anyway, no solutions, but it is awkward and tbh they CANNOT give your job away, you are entitled to keep it, you can in fact talk to HR or Personnelle or whomever and point out what was done was in fact illegal - maybe the CAB or something could help? You can choose to come back to flexi-time but they gave your job away while you were on maternity leave, which is not allowed unless you said you didn't want the job anymore, which it doesn't sound like that happened.