1 point 5 litre Bottle of Brut Dargent 1986.?

I have a bottle of aforementioned Brut Dargent ,Methode Champenoise,Blanc de Blancs, Chardonay 1986.I have it since 1988 and kept it on its side in a cupboard in a relatively cool room off the kitchen. Can I have comments please on its condition,suitability for drinking, especially for a family celebration soon.