Opposite the train station. Be ready to pay to visit the house, pay to walk around, pay to see the animals... There is a nice court with nice food. Also a nice free play area for kids.
Hatfield house, specifically the long gallery, served as the headquarters of the ‘Temple of the Four Orders’ in Sherlock Holmes (2009), where Holmes meets with Sir Thomas and the Home Secretary.
Go to Battle Proms in July. It's wonderful. Take a table, chairs, wine, a full on buffet, strawberries, lanterns and cake, cos everyone does & you'll feel left out if you don't.
Bring a picnic. The prices here are crazy. Also: they've ruined it by bolting the kids' play area onto the farm - thus forcing you to pay to get in. There's a separate price on *everything*. Meh.