Firstly, Dalkey Dragon and Becky - you are too kind! Google is my friend!
Second - Becky, you say the pension scheme 'was' defined benefit! I thought it still is?
At the time of the introduction of the 1977 Scheme, Widows and Orphans Scheme etc, there was much publicity and internal awareness programmes, although this may have differed from public sector employer to employer. Also, unions were active in advising their members on the pros and cons of the new schemes. Many men chose not to sign up for additional deductions(and men made up the majority), could not envision dying and believed the state would take care of their widows and children.
However, if your HR file does not show a form where you chose not to opt for the new scheme, you may have a case to appeal on the grounds that you were not given the opportunity to opt in.