UK passport/citizenship qualification question

hey there, ive searched but have not found so much useful information.
my mother, originally a british citizen got her canadian citizenship in june or so of 1974. i was born 1975. all the info i can find suggests the mother has to be a british citizen at time of childs birth- however, she never renounced her brit citizenship when she became a canadian citizen. does anyone know the rules of dual citizenship back then, or what kind of communication would have taken place (if any) between canada and britain? she doesn't know if she was still officially a british citizen at the time of my birth (although she was a 'subject'-but that would only help with the right of/to abode).
the reason-i want to understand if i even qualify for citizenship (which would open up the EU to me) before paying the $$$ for submitting an application.
thanks!
graham