In the UK up until January 1st 1980 they used the % Proof system for measuring the strength of alcohol. They changed it in 1980 to the system we still use today the percentage of Alcohol by Volume (%ABV)
Therefore if this was bottled for the UK market it was bottled before 1980. If it says 70% Proof on the label on the front of the bottle i suspect it may be from the 1960's or possibly 1970's. If so the shop price for for this would be around £60 - £100, in a private sale you would only get around half this amount £30 - £50
If you have a look over at The Whisky Exchange they have a few bottles of Dimple, they may have one similar to yours.
It wont make you a fortune, if it has been in the family all these years then i would say drink it and enjoy it.
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