I have oak widow sills that have been weathered for 150 years. They are deeply crevassed and painting them is not a viable option.
I don't wish to replace them but would prefer to coat them in a layer of sand in a resin binding to give an imitation sandstone effect.
What would be the best adhesive and binding agent?

It's not really a composites application, so I doubt Easy Composites are really going to have anything suitable. They would have resins that could work, but for the application they won't be ideal - I'll let Rich confirm, but that's my suspicion. 

The resins we use in composites are really not suited to long-term outdoor exposure at all. You need a resin for the constructing or landscaping industry, I know there are single pack polyurethanes being used for pebble and stone binding agents but wouldn't know where you would look for them.