Grant, 19, hit a post early on, before having a shot deflected in on the stroke of half time via Ryan Willams.
Grant then capped a tremendous personal performance by netting 10 minutes from time as Terry Brown's side moved into third place in the Nationwide Conference National table with a convincing four goal victory over the Hertfordshire outfit.
Grant who has already made 10 first team appearances for Watford is on loan at the Hampshire club until the end of the year.