Nikon not honoring Guarantee

I bought Nikon binoculars (at a premium price) with 20 year guarantee for my wife 11 years ago. She used it once a year to watch whales. After 11 years the one lens became loose. Miraculously I still had the original invoice with guarantee and was very relieved to be able to replace/fix it. I was shocked that the local South African agents Premium Brand Distributors walked away from any responsibility claiming "normal wear and tear" for an expensive item with a 20 year guarantee that is used once a year. To top it I disputed the original claim and it was just ignored again. I finally gave in and ask the dealer to get it fixed upon which I was faced with a revised quotation of 25% more. This is **************** . I am so upset.