I assume....
Thousands of people already have your PPS number, all the public servants who handle them daily, anyone you've worked for, and any accounting staff in those companies. Since so people many get them wrong, or try to use other peoples, they have to cross check PPS number with other details and all mail, payments associated with it, will go to the address that social welfare have. PPS numbers are unique so you can only have one address associated with them. So someone else can't use your PPS with another address. That said theres always the possibility of identity theft, as the recent event with peoples passports being cloned illustrates. I would imagine thats very rare though.
Try emailing them or writing a letter.
Maybe some unions might still be on a work to rule?