Thanks for all your suggestions. I'll start with salt and some beer traps. Copper tape around the box of straw. I've not got many coffee grounds as I'm a typical tea drinking Brit ☕, but if this goes on I'll cultivate a coffee habit. 😂
The corner of the yard where I keep my quail hutches is CRAWLING with slugs. Dozens. I went out there to check on them and it was nightmarish. The floor is coated with slime in the mornings.
Is this a thing? Are they after the straw? Am I going to have to poison them? I don't mind the odd slug...