If you see any unusual or unexpected plants or animals on Heritage Open Day (or any other time for that matter), why not tell everyone about it by adding your sighting to the BRLSI website. Just select Local Studies and Wildlife - Add Sighting. If you do not know what it is, no matter, there will be an expert on hand from 10am to identify it for you.
They will then email you a link and you, and all your friends, will be able to see what it is. You can see what everyone else has seen recently by selecting Local Studies and Wildlife - Recent Sightings.
You can use your phone or a computer to add sightings and can add as many photos as you like. You will need to supply a grid reference, which you can find on the edge of OS and A-Z maps or if in doubt, just enter 7 as the 'Easting' (reading on the top or bottom of a map) and 6 as the 'Northing' (on the side of a map).
PS. You will need to identify some pictures yourself, just to prove you are not a robot!
By the way, the creature in this picture is a Wasp Spider, photographed by Mike Williams. It has only recently been found in the Bath area.