The WWII perimeter track is in dotted lines on the map I sent to Collectair...the section parallel to The Vale was dug out in the mid fifties so I think it is correct car trials could only be done as far as the kink by the Berkeley School playing field....I can only personally remember the slow speed manouevering competitions which took place in front of the hangars on the wider tarmac (sorry the map didn't attach)try the link
Heston Airport on Flickr - Photo Sharing!