At this event Aprilia ENA400 was being driven by and probably owned by Midge Wilby, who was a well known northern competitor. She is mostly associated with the Atalanta - see
http://www.atalantamotors.co.uk/template-98.php?page=29§ion=7 . A few years ago I tried to help an Aprilia owner/ restorer, who was keen to find out more about Midge Wilby's Aprilia, but I couldn't find any information on ENA400 in any of the LMC records that I had access to.
Colin