I am now wondering what attracted whatever laid the eggs that became the maggots. Dead rat or two perhaps?
Your description of how damp it was again leaves me wondering. I am intrigued, was the main contents perhaps badly rotted brassica stems or grass.
Sounds like whatever was in there was mainly green stuff with not a lot of soil or other matter added
As for introducing maggots to dispose of slugs and snails I think I would prefer the resultant large slugs or snails to have to deal with.
Maggots have a habit of producing flies
Just remembered my fishing days, used to keep maggots in a bait box and if I didn't add some bran or sawdust after a couple of days the whole lot became a slithering slimy mass from where they sweated.
Typing this at breakfast time, anybody been put off their morning bran