DMP question (sorry I can’t find a DMP forum)

Hi

I’m on a DMP with StepChange and have been since January. All of my creditors besides TSB have accepted and are pretty much leaving me alone, TSB are sending me letter after letter after letter. I’ve had another one today saying they won’t accept payment unless it’s received on the 1st of the month, they said this back in February and I’d contacted them to say that that StepChange only pay by the 28th regardless of when I pay them. I cannot get them to pay any other day. I logged a complaint which they’ve replied to saying they aren’t in the wrong and they will not accept the plan unless it’s paid on the 1st. What can I do? I’ll ring StepChange again in the morning but I’m at my wits end with TSB. We live at DHs work and all my post goes there, it’s pretty obvious I’m getting around 7 letters a week from a bank which looks a bit suss to his reception staff. I’m 6 months pregnant and trying to do the right thing with this debt. I don’t understand why they’re making it so hard.