This one is Segestria florentina, the largest of our Tube Web spiders. It is another introduction through shipped goods, this one being from the Mediterranean, and is now well established in the southeast and the southwest - hotspots being London, Plymouth and Bristol.
They are one of the few UK spiders that are able to bite but with no lasting symptoms. They can be aggressive if you poke fingers at them, but as you have found they would much rather run away. Have a look on walls and fence panels for them sitting in their tube webs, particularly at night with a torch and you can see their iridescent green chelicerae shining back at you.