Additional evidence

I apologise if this is a stupid question but I've already completed and returned my husbands PIP form then realised I stupidly did not put in some evidence I had intended to. I remember reading on the form itself that some evidence can and may be needed to be sent in after the original form has already been sent but I do not have a copy of the address (nor can I be certain of it using google) of where to send it. I have tried three times to get through to the PIP helpline, spending 74 minutes in total so far now just sat in a queue with no success. Would anyone on here be able to advise me of the address to send a covering note with additional evidence attached please?