Thankyou you two.
I can't fish it out immediately as I wouldn't be able to keep getting up each time one of the three cats went to toilet.
The smell isn't nice (obviously) but they're good at covering it up which helps a lot.
I just had an image of a persian refusing to use a litter tray because it hadn't been poo picked and wanted to check that they don't tend to be fussier than most other cats.
Should I be ok?

Edit: just noticed I posted this from my friend's account whoops.