Seems like the DWP is full of them !
Finally got some (relative) sense out of them today, after another 70 minutes on the 'phone, to two different sections of the same Department, each of which didn't know its Rs from its elbow, and actually got a payment of Pension Credit into my bank account.
I only got this by being politely forceful, as each section stated that all was in order on their part, and I had to contact the other section in each case !
It's still not completely sorted, as there are still substantial arrears to be paid, but at least I now know what I should be getting in the future, although I was told I need to contact the relevant section to inform them that the other section has now adjusted things correctly.
No more 'phone calls from me, getting the 'run around' again - it's now all in writing, laying out the whole sorry catalogue of errors by the two sections of the same department, and politely, but forcefully, requesting an immediate response, with full explanation of and confirmation of what is still owed, followed by immediate payment into my bank account.
This route will probably take a little longer, as my letter will be passed from pillar to post, but at least I know that it will generate a response, and quickly, as a copy is also going to the relevant Minister at central Government !!