Looks like an ink cap - if it is, they're not exactly poisonous, but people do get a reaction to them if they consume alcohol at the same time. The gills inside should look crowded, white at first, changing from that to brown to a dark, inky mass. If you're worried, nip out in the mornings and just remove them all.

Thank you very much Bamboo. I've had this confirmed elsewhere but was waiting for another comment to the same effect. I've been removing them but at 5.30am it's like an undercover operation in the dark - LOL! It's put my mind at rest.