Something 'orrible in the 'olly'ocks!

While pricking out the hollyhock seedlings I found this little guy [shivers] hiding in the seed compost. The 'baby' hollyhocks didn't appear to mind as I don't think they've been damaged. But where did it come from? What's it been eating? What is it? And do I need to open the war chest?
Thanks for any help.

Thanks for any help.

Trying to be the person my dog thinks I am!
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Doing a bit of research it looks like some sort of cut grub, but I've no idea how it got into a seed tray.