It was at Wollaton Park, Nottingham, in September 1979.
According to the Bord's "Modern Mysteries of Britain" (1987):
...four children aged between eight and ten claimed to have seen about sixty Little People riding around in little red and white cars. Angela Elliott described what happened:
We heard this little tinkly bell. We started running and these little men came out of the bushes. There were about 60 of them in 30 cars like bubblecars. They were half my size and looked old.They had greenish faces with crinkles in themand long white beards with a bit of red on the end. They were laughing in a funny way and driving over swamps near the lake. We were frightened and ran to the gate. I don't think they liked the lights outside because they didn't follow us to the street.
The section goes on to describe the Little People as wearing yellow trousers, blue tops and caps with bobbles on them like nightcaps. Angela said they'd seen them before, in the summer, and despite their fear the Little People seemed "friendly and joyful".
There's another report, from Kilkhampton in Cornwall, of a little man driving around someone's garden in a tiny red car. This was about 1940. The description given is extremely similar to the Wollaton Park one. And a little man in a little plane, complete with flying helmet, in Hertford in 1929.
How's that?