Hey there Andy!
In order to help determine the age of your bottle, you will need to send Cutty Sark a digital image of it to:
info@cutty-sark.com You can also give them the all the details you have given us. However, please keep in mind that Cutty Sark will not be able to give you a market valuation because the Scotch whisky only matures while in the cask and never in the bottle. In addition, Cutty Sark states that the life of a full and 'sealed' bottled of Scotch is indeterminable. "A quality like Cutty Sark is there to be savoured; it will not deteriorate if kept unopened in the appropriate conditions."
What are 'appropriate conditions'? Here are guidelines:
1. If the scotch came in a 'presentation type' box - keep it in there - this box is not only for esthetics, but designed to protect the whisky
2. Keep your whisky in a cupboard - or liquor cabinet. Key is to keep it OUT of the sunlight
3. Temperature - what's ideal? First, don't expose it to extremes of temperatures and second, keep it at what's known as 'cellar temperature' - which is between 65 and 67 degrees Fahrenheit, or 18.3 - 19.4 Celsius
4. Store whisky in an upright position - you want to keep the Scotch AWAY from contact with the cork.
Best of luck to you!