thanks for the help guys, I knew it had to be sometime during the 70's as that was when I worked in the chemist counter in that well known high street chemist, but wanted a more precise answer in order to follow up my pet hate.
I do remember when we first got one of these medicine bottles and in the name of science - whilst fully supervised - we gave our daughter the bottle and said 'open it' which she did quicker than I can.
From memory she would have been 5 or 6 which means we are talking 1977/8