“If a man makes substantial sums of money in betting - and a number of people do so - it is not unreasonable to expect him to keep records of his betting transactions so that, if he is subsequently challenged by the revenue authorities to explain an increase in his wealth, he can satisfy them that it is not due to any undisclosed taxable profits but to his betting winnings. If he chooses not to do that, he runs the risk of having attributed to taxable profits what, if he kept records of his transactions, he might have been able to convince the authorities were in fact untaxable betting winnings.”