Daddy Long-legs Spider Pholcus phalangioides
I always have these in the sheds, and garage, and they will also appear in the house. I leave them be in the sheds beacause they are not hurting anyone there or causing any bother, but in the house they have to go outside. If you do see one of these and get too close to it whilst it is dangling upside down in its web it will vibrate quite madly, a way of confusing and putting off predators. It is non-native to Britain, most likely arriving here in imported goods, but it has now become well-established over the past 30 odd years.