Hi New Guy,
I have been doing a bit of online research after reading your query, and have to admit, they're certainly not making it easy for you. You look like you've tried the options available to you.
Maybe's a bit of a silly question, but are you able to ask anyone who can call them on your behalf, and write down what you're wanting ?
I could only find the freephone number and link you've already supplied (
secure.dwp.gov.uk/get-a-proof-of-benefit-letter/ ) as a way of contacting them also.
Sometimes as difficult as it is, were the ones who have to make the adjustments in order to contact them, when they should have services like an online webchat available for example.
Regards, Chris.