If you don't have any sand on hand, you can try filling the bottle with dilute vinegar and leaving it to sit overnight
. This will attack a lot of the gunk. Then fill the bottle with warm water and your choice of detergent and give the bottle a ruddy good shaking, holding onto it very securely. The motion of the water should flush off any remaining grime. And the real beauty of water for cleaning is that it can go into even the tiniest crevice.