It is seriously a lot harder when the other parent is still in the frame.

My DH has the same rights as me - we are a united front. I think that is vital and what a Step Parent should be given, else they just become the glorified (and free) babysitter. But my XH isnt here rocking the boat about it.

Your DP needs to be a united front with you too. Without that, one day you may run into the "you're not my..." rebuttal because they wont see that your actions are being approved by someone who is!