Thanks a lot!
I have also verified this at Diageo.
Next to giving the same age estimation, Diageo also told me that:
"Whisky is in its peak condition when bottled and does not mature or get better once in the bottle. It is generally believed to have a shelf life of between 10 and 15 years after which the colour and taste may begin to deteriorate, particularly if there has been any evaporation from the bottle. As your bottle is older than this then I would recommend that, if you do intend to try to drink the whisky, you nose the contents well on opening to check that it smells ok."
It doesn't sound as if it's a good idea to keep this in my cupboard for much longer if i intend to drink it.
Maybe the best would be to just put it somewhere on a shelf for display as the taste might already have deteriorated since there has indeed been some evaporation.
Would you think it still tastes good after 45-50 years?