Anything further I can do?

I went for an ESA tribunal at the end of July and passed it, am now supposed to be in the WRAG group.
My problem is; I am not entitled to any actual money (was getting contribution based before and have exhausted my entitlement to that ages ago) BUT should be getting NI credits paid (which is the reason I decided it was worth going to tribunal)
Now, despite chasing it up TWICE by phone with DWP I STILL haven't had an official letter saying that I am now in the WRAG group. Apparently there's "no timeframe" for getting this done although they have definitely received the notification that I passed the tribunal. And until it is done, and the letter sent to me, I'm not getting my NI credits paid either (though I was told this will be backdated)
It's really annoying me now, it's been 3 months since I was successful at the tribunal! Apart from phoning yet again (with all the hanging on the line for ages we all know this entails) is there anything else I can do to chase this up??

Thankyou for the reply Gordon. I do know I'm not entitled to income related. I've been assuming I'm at the back of the DWP queue because no actual payments are involved, only the credits.

I think I might try contacting the DWP one more time, then I'll try my MP if I have have no luck (mind you, he didn't reply last time I contacted him - nothing to do with DWP though, I was campaigning on behalf of some cause or other!)