Our contact at Bell's parent company told us that from looking at the images of your bottle, they would say it dates from the late 1970s. The bottle features the 70° Proof alcoholic volume statement; this is the equivalent of today’s 40%ABV, and was discontinued as a unit of measurement in the UK in January 1980. Your bottle looks just like the Bell’s bottle that was on sale in 1979 before this change took effect, however it could have been produced any time between 1975 and 1979, when these labels were being used. Unfortunately we can’t narrow the date down any further than this.
Interestingly the bell shaped neck tag around the neck of your bottle was part of a promotion called ‘Other Famous Bell’s, which ran between 1984 and 1987. There were 42 of these neck tags each featuring a different famous bell, and once consumer had collected all 42, they could send away for a presentation booklet to keep them in, as well as various prizes. Since they're not sure if you’ve had the bottle since it was purchased or if you’ve inherited it, but they guess it would be that the tag has come from another later bottle.
Hope this helps! Jojo