It's the other side of Avenham Park. To get there by car from town centre, drive down fishergate hill with the county hall to your right, then you turn left, but I always take the wrong left turning, lol. You go past the big fancy temple to get there, if that is any help.
I can second it being brill. The only problem is, it's a bit hard to get to, but in the summer, the beer garden is brill - easily the best in Preston, and full of kids. They have arty events on too, and good beer, AND nice non-alcoholic drinks, which is a rarity.