Thanks to Jeff Pitcher for the postcard showing sailing on Tilgate Lake, not sure what the date would be 70s? I took the below current day one the other weekend when Dynamic Adventures had their boats out.
Found the below images on the Britain from Above website. If you use the link below to their website you can actually zoom in really close and get some great views of Tilgate Park. https://britainfromabove.org.uk/en/search?keywords=tilgate&country=global&year=all