Tony McCoy will take the ride on Sir Des Champs in the Betfred Cheltenham Gold Cup. His intended mount Sunnyhillboy was declared a late non-runner by trainer Jonjo O'Neill.
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A top temperature today of 13 Celsius (55F).
Mr Trump tweeted that Mr McMaster has done "an outstanding job & will always remain my friend." He said MrBolton will take over on April 9.
The committee wants a more stringent interpretation of air quality laws and for noise estimates to reflect a range of possible flight paths.