There has been a bunch of news since this case came up. Just a month or so ago a women found a perfume bottle in a park with poison in it. She has since died. This event helped them identify some suspects on CCTV cameras. They are Russians. I don't think it's hard to believe that Russia would try to kill Skripal. They consider him a traitor to their country. You can read more details about what's new here: https://www.theguardian.com/uk-news/2018/jul/19/police-identify-skripal-novichok-salisbury-poisoning-suspects