I trained at Panshanger Aerodrome (North London Flying School) about 7 years ago and still fly there as an aircraft (group share) owner. Their website has lots of pictures and prices etc....
Panshanger was the WWII decoy for the old Hatfield aerodrome De Havilland facility, so thats close to the area you are enquiring about. For PPL training, They have about 7 Piper Cherokee and Archers to choose from, and a Grob. It has a nice long grass runway (about 875m) and a well resourced club house. I found the training to be very thorough and the instructors, staff and other pilots are extremely helpful.