Redundancy Pay - Late

MumofCR

I took voluntary redundancy at the end of last month and my last day was 30th April. In the letter that I signed when I agreed to the redundancy it stated that I would be paid monies owing to me within 28 days. As of yet I haven't received anything. I spoke to the HR department this morning and apparently they haven't received the redundancy paperwork from my line manager. Strangely enough I didn't get paid this month so they most have known something about it. I've spoken to my previous line manager and she says that she's done everything that she's supposed to but will chase it for me.
Luckily I don't need the money just yet but chances are it's going to be at least another month before I get a payment. Is there anything that I can do as they have broken the redundancy terms that I signed up to? I'm a bit fed up that I will be missing out on a months worth of interest (every little helps!) on my redundancy money.