Thanks for the answer
@BigBill. I'm in London which is, according to Google, "humid temperate oceanic climate".
This plant never drops its leaves. It's been having these holes for a long time but I think it's increased over time.
It could be a slug/snail problem yes, but am I right in thinking that the lower leaves would have been damaged more because slugs/snails could reach them more easily? In my case, almost all parts of the plant are affected similarly.
