The dog saw it a moment before I did, and almost managed to grab it off the vinyl floor before my quick intervention. This spider was BIG, appearing significantly larger than spiders usually found around the house, and with much bulkier body and legs than the long-legged harvestmen we sometimes see. Almost scary!
I first made a quick grab for the camera before it disappeared, and then carefully extracted it to the outdoors, using a large tumbler and piece of card.
An unusual feature of this spider is that the thorax is larger than the abdomen – most seem to be proportioned the other way around. Research shows it to be a giant house spider, actually quite common, with body length over half-an-inch.
According to Wikipedia, these spiders have eight rather ineffective eyes; the flash used for the photo has highlighted two of them. These spiders are not said to be keen on biting pets or people, so are not as scary as they look at first sight.