Let's have a look at which players did the most to earn wins/losses/draws for their team:
Cook and Prior were England's standout players on Day 1. Cook remained calm as three quick wickets fell early in the day; his impressive knock of 96 was worth 10.2 % of a win for England. Prior's rapid knock of 73 left him unbeaten at the close of play with 13.1% of a win to his name. He has also done more than Cook to take the game away from Sri Lanka, lowering their chance of victory by 17 %. Lakmal's three top order scalps made him the early WPA front runner on the Sri Lankan team; his effort was worth 12.4 % of a Sri Lanka win.
Here's a graph of the game's progress as of the Day 1 close of play: