After Mahmoud Bhatti had given them a fifth minute lead it was only saves by goalkeeper Luckes at five penalty corners which kept Grinstead in the game.
However, when Ben Welsh followed up a Barnes penalty corner shot in the 26th minute the game was beyond Hounslow's reach.
The second half, however, belonged to Hounslow with Brett Gillman pulling a goal back after 19 minutes. With just a minute left Graham Ortleib put the game beyond doubt.
Surbiton took the Bronze medal beating Brooklands 4 - 2 with South Africa's Greg Nicoll scoring a hat-trick but 17-year-old Joby Smith was the architect of their victory.
Jolein Abbas, the Dutch captain, with two second half goals inside four minutes, assured her side the Gold medal in the Women's Junior World Cup final in Seoul in a comfortable win against Australia.
Argentina, the defending champions, beat Germany 3-1 to take the Bronze medal.
England's 4-2 win in a penalty shoot-out against China the previous day after a 4-4 draw won them seventh place - their highest placing ever in the World Cup.Reuse content