I started a thread on the forum as I keep finding droppings on, and around my bins (https://www.birdforum.net/showthread.php?t=388444). With help from this forum, I have finally worked out what is leaving the droppings. It turns out that they are from caterpillars!
I was wondering if someone with more knowledge than me can ID the type of moth (presuming it is a moth) that this will grow in to?
Pictures attached. More pictures available.