Yes, this is Steatoda nobilis. They can often be found in cracks in brickwork, under sills & around window frames...anywhere where they have some cover. They're perfectly able to survive outside, so if they have a hiding place & a web to hang out in, they are unlikely to try to get indoors. Unless driven off by outside forces, they tend to stick close to home & not wander far (a few inches, rather than whole feet/yards).
Thanks, I was pretty certain it was. My father-in-law found it when he was re-potting the hanging baskets, there are lots of tangled webs up under the fascia boards which where I suspect it lives - and has done happily for a while!
Not seen on in the house yet, but I will be sure to check the markings of anything now I know - especially as we have a baby.