ooooh I get this stuff too, it is quite shiny and rubbery looking, almost looks like slugs but it isn't. I know it is not worm casts or any sort of animal poo as there is just too much of it and it covers a large area.
I have been wondering about what it was so I am glad someone else has it too.
Are these "deposits" jelly like ? Do they look like slimey round bits of seaweed ? If so, they're likely to be a type of algea caused by poor drainage, compaction and/or low light conditions
in my case it sounds like Hammermans description and it is algae.
thanks from me to Penwern for raising the subject and Hammerman for telling me what it is.
I did not think it was animal poo as there was too much spread over a large area overnight.
Hammerman's answer seems the most likely. It looks a bit like the hedgehog poo, but it is in smaller bits and so much of it. I wondered if it had any connection with frogs, as I have several in my pond/