For anyone looking to have a bet in the national if you eliminate all horses based on the following set of rules you would only have missed, as I recall, 3 of the last 20 winners.
Eliminate
Any horse out of the handicap
Any horse carrying more than 11st 5lb
Any horse without a run in the last 52 days
Any horse that has run in the last 13 days
Any horse wearing blinkers or a visor
Any horse not between 8 and 12
Any horse that has failed to win this season
Any horse that has fallen this season
Any horse that has not been placed in the first 3 this season
Any horse that has not won over more than 3 miles
Any horse that is not been out of novice company for 2 seasons
Any mares
Any previous winners of the race
Any horse ridden by a lady
Any horse that's ratibg its based on rings on a foriegn track.
This will give you a list of horses that fit the national profile and very little chance of eliminating the winner.
I would also add, for future reference eliminate this year's gold Cup winner.