They cannot force you not to take it if you have an agreement in place but they may well ask you not to continue with it and if you are not keen on that idea, they may try to pressurize you into it by saying you are really busy. Did you agree in writing (email counts) – if so, get that out and have it at the ready.
I would advise trying to keep things as amicable as possible though, you do not want to get into a difficult or stressful situation with them.
Check the maternity act site for more details or talk to some one in HR quietly about it, in confidence, before your meeting if you can. They can advise you.
Hope some of that helps. Best of luck.