Hard to tell from your photos.
We do have a few in the
Gallery that you can compare them to though:
Steatoda nobilis 
Photo by Demicav
Steatoda bipunctata 
Photo by UB4 gardener
External sites:
Steatoda grossa photos and info Steatoda nobilis photos and info Steatoda bipunctata photos and info
Unless you're planning on turning your loft into a room I'd leave them be.
S.nobilis can bite, but only in self defence - just give them the same respect as you would a wasp or a bee and you'll be fine.