Under the lid of the water butt in the garden I found this small Tegenaria, guarding its curious egg sac. I had never seen a Tegenaria egg sac before, the reason being that, according to W.S. Bristowe, their egg sacs are normally attached inside the spider's funnel, where they would be hard to see. In this case I probably broke the funnel when opening the lid and the egg sac seems to hang from the plastic by a few threads of silk. This peculiarity and its small size compared to other Tegenaria specimens make me think it is T. domestica, a species found in or outside houses. I hope I didn't disturb the spider too much, as I plan to follow the development of her spiderlings, which stay with their mums for a few molts inside her funnel.
UPDATE 9/06/2014. I now believe the egg sac belongs to a spider of the genus Ero. See this BugGuide thread for more info.