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You can narrow it down roughly. You know the difference between '90 spec and pre '90 spec? Obviously late 1989 and 1990 had a lighter grey interior and a wider spoiler with downturns on the end, amongst a list of other differences. This isn't 100% reliable, because parts can have been added over the years.
check the front bumper, spoiler, interior, or you could always find your nearest halfords and give them the reg and they will give you the details of the whole car, i.e year month, where it was made etc
it sounds like an 80 spec to me cos its very rare for someone to put an 80 spec spoiler on a 90 spec car, the 90spec front bumper has a more open mouth on it