I didn't think you had to have another twelve months of service in between returning to work and going on mat leave. That's more like leaving your employer and going back again.
I would have thought you should still have continuity of service.
Did you hve anything in writing about going back early?
My thoughts too. My GF (although FT permanent) went on Mat leave & ended up pg with # 2 whilst on Mat leave was still able to take Mat leave again for 12 months b/c she has more than 12 months continuous service with the company overall, not from when she returned from her original mat leave (if that makes sense)