She is a Steatoda nobilis, AKA the false widow. These are very common spiders where I live and are often found in and around houses, offices, etc. They are able to deliver a bite which, though undoubtedly unpleasant (symptoms may persist for a couple of days), is not dangerous. I once stayed in a resort on the Isle of Wight and the chalet bathroom had several living in webs around the windows, I currently have a female living in a vent in my living room window. The females are docile & quite skittish. They normally stick close to their web/bolt-hole, I wonder why yours has gone walkabout, they don't seem to like being on the ground/floor. If she finds a suitable nook/cranny, high up, she may set up home there & stay put. My experience is that they are quite territorial.
The males have much smaller abdomens and tend to be a little drab in comparison, still with a similar marking. They wander about a little more looking for mates, I'd be inclind to remove these to the outside.
They are not aggressive and bites are extremely rare considering how often they are found in our homes. I can understand you perhaps not being keen on her presence & maybe wanting her gone, but in reality there is little to worry about. She will probably be just as happy outside, on an exterior wall, if there is somewhere she can hide.
thanks for responding so quickly and the detailed information. That's al very useful information. I found her on the floor after the big tunderstorms on Monday. I thought that might have had an effect on her. She seeked higher ground this moring in my living room but I first saw her in the bath. If she stays at top levels I'll let her be. But otherwise I'll try to relocate her to the outside.