His agent for books and cartoons is Gordon Wise who can be found here.
Gordon also looks after the literary estate of Winston Churchill, so he's very much a one-stop shop for anyone wanting both cartoonists and wartime Prime Ministers.
His agent for scriptwriting is Ben Hall who can be found here.
Their other details are:
Curtis Brown Group Ltd
Haymarket House
28 - 29 Haymarket
London SW1Y 4SP
His books are all published by Hodder and Stoughton, 338 Euston Road, London NW1 3BH. His editor there is Jack Fogg and his publicity is handled by Katie Davison.