SMP missing 3rd mistake in as many months.

[calibee]

I posted a while ago regarding mess ups with my maternity pay.
Long story short... Despite me doing all the right things and making calls to make sure HR knew I wasn't returning after the birth of my twins last November, they failed to inform payroll and I ended up with an overpayment.
After many many calls I have set up a direct debit to pay back this amount out of my remaining SMP each month. I officially leave this month. Despite setting this up with a member of the Overpayments team ( I have letter and direct debit mandates copies) they still proceeded ( without any discussion with me) to set up a loan of £80 monthly, and begin to take it out of Mays salary.... Even messing up again by making the loan as if I was returning to work.
So, this was sorted ( or so I thought) and was expecting the £80 returned in June's pay packet.
Lo and behold... No repayment. Adding insult to injury I haven't even received my SMP!!
I have been calling since last Thursday, being passed from one department to another. The lady who promised to get back to me on Friday and didn't (from maternity) is now off today. Meanwhile, the direct debit of £200 from my arrangement with overpayments has come out as well as other bills and I am now overdrawn :(.
This morning I have asked for a manager to call me back... I am still waiting.
How do I escalate this???
I am now out of pocket,and there seems no rush from anybody to help.
It feels so unfair when the mistake wasn't even mine in the first place :(
Can anybody advise please?