U.K mums - return to work - ALREADY part time!

So - it says in the gov guidelines that if you return to work after additional maternity leave that you may not be offered the same job ... BUT will be offered a similar job with the same terms and conditions. 
I'm already part time and just want to KEEP the same hours - not even necessarily my position - I'm a teachers assistant in a primary school so chances are my class would change anyway. 
I can't find Info ANYWHERE on whether you can return to your same hours and shifts.  all the info is about reducing hours and them turning it down. I want to keep working the same hours I already do on returning - in the mornings. And change nothing - can they contest the hours they've already agreed to?
Many thanks!