You can get a pension forecast instantly online if you already have a username and password for the Government Gateway issued by HMRC that lets you file your annual tax return online.
Simply go to
www.thepensionservice.gov.uk/resourcecentre/e-services/home.asp follow the links and use your Government Gateway username and password. This shows you how many years pension contributions the government already considers that you have made and any years you have missed that you are entitled to buy.
However the Pensions Advisory Service is in my experience a largely useless outfit where the staff are clueless and do not even know about you being able to get the above actual Pensions Forecast and how many years contributions you have made online......