Confidence: It might be this.
Notes: When I've seen rat-tailed maggots they've been in water, or at least in a very wet environment, and look paler, more translucent than this, and the tail is a bit thicker. So my suggestion is no more than a desperate guess. These live in wet situations with high organic matter like water-butts with lots of leaves, or ditches with slurry. They don't mind anoxic environments because they breathe through their 'tails'. The adult is the hoverfly called the Drone Fly or its relatives.