DAVID`S LAD SUBJECT OF A SUBSTANTIAL NATIONAL WAGER David`s Lad has been backed at 20/1 to the tune of Stg#25,000 to win next year`s Martell Grand National at Aintree on April 5th.The eight-year-old, trained by Tony Martin, is now 16/1 for Aintree glory with one particular customer standing to win #500,000 on the strength of that bet. 'This has to be the biggest horse racing ante-post take-out of the year,' said Balthazar Fabricius of Ladbrokes.'Taking bets of #25,000 or more is nothing new for us but given the odds, we do face the one of the bigger pay-outs that we have seen for a while,' he added.Martin`s charge was still travelling well under Timmy Murphy when crashing out at the fourth last in this year`s race won by Bindaree.