It might not be something you want to mention in the Half Moon Inn in Balcombe, or around any of the other communities where people are getting anxious about shale gas explorers ripping up the countryside with their drills and pipelines. But if shale is the tremendous source of wealth that David Cameron insists it can be for this country, how do you go about investing it? After all, if there are fortunes to be made, there is no reason not to claim your share.