Hi Souter
I am surprised that PC department did find your old pension when you first applied as they have access to most things.
Your private pension will affect your pension credit, you may be hit with an overpayment depending on the value of the pension.
Whoever told you that you can have a certain amount without it affecting pension credit is wrong, all incomes affects Guarantee Pension Credit.
I would advise you to seek help from your local Welfare Rights Organisation;
advicelocal.uk .
Gary
