Chris, Hurst Park racecourse closed in the early 60s, but the gates are still in place, near the river.
I was a member and user of Molesey Boat Club for around 50 years from a teenager until around 2000 and the access road to the club crossed a preserved section of Hurst Park Racecourse which I believe was a Start.
Regarding Northolt Park Racecourse, I had no idea one ever existed. Its heyday was obviously the immediate pre-war period. When reading its history earlier today, it appears there were abortive plans in the early 50s to transfer the pony racing activities from Northolt Park to Hawthorn Hill near Maidenhead. I recall there was an airfield there attached to a beautiful manor house (owned by Peter Cadbury??) used for commercial aviation during the 50s used by a couple of small feeder airlines.
Chris