Length 1-2mm. This has to be the smallest spider I have ever encountered. I found it on my bathroom wall early this morning, and it is called Oonops domesticus also known as the Goblin Spider. It was so small and delicate I had to use toilet paper to catch it, believe it or not. For such a small creature it moved very rapidly and was hard to keep track of. After taking these photos I lost it under the kitchen table somewhere, and I doubt I will ever find it again. Because it was raining I had shaped a leaf to fit the bottom of a small pot so I could contain it and get a natural setting for the photo, and took the images on my kitchen table.
It belongs to a family of spiders called Oonipidae, of which there is one genus, and 2 British species. Oonops pulcher is the other species, and is generally found under bark, stones and leaves, where Oonops domesticus is found in buildings. To accurately identify them you would need a microscope, although Oonops domesticus has five tibial spine pairs where Oonops pulcher has four, and they do live in distinctly different habitats.
It has six tightly clustered eyes, and is a creeping, stealthy hunter of small invertebrates, interspersed with rapid movement. It apparently gently strokes its prey with outstretched legs before darting forward and biting it. Because of its size and nocturnal habit it usually goes unnoticed in houses, and is probably under recorded. During the day it remains hidden in a silken cell behind furniture or in cracks in woodwork. It is widely scattered and uncommon in England.