Another vote for conkers, something to do with a chemical they don't like.
It's actually Chestnuts not Horse Chestnuts (conkers) that they hate.
There is a local spider problem around here owing to living next to a large lake. Local shops stock a spray "No more spiders" - contains Chestnut Oil and Clove Oil, doesn't kill them but wards them off. Apparently they don't like citrus smells either so things like Pledge polish work around the places they come in.



