Amazing to find some recollection of motorsport at Heston. Yes, what I saw was on a section of peri track that ended near the top of the school playing field. It was a section that involved a U turn where the track ended and ruining back towards the start.
I have always been puzzled by an odd bit of tarmac track that still exists... could this be the U turn to which you refer???
see
The Airfields Of Greater London Heston and zoom into where the M4 passes under North Hyde Lane...