Small insects, baby frogs, woodlice etc.
However, it will soon need to hibernate, so don't keep it indoors or it will starve before spring.
Better still keep it a few days then let it go where you found it.
Mine was called Marmaduke.
Thanks, New Forester, for your speedy advice.
It was my mother who accidentally brought it into her conservatory with, she thinks, some compost for the plants from the garden. She found it working its way across the slate floor !
I'll let her know to return it to the compost heap. Should she try and feed it some woodlice, and so on, first ?
p.s. hers is just called snakey, apparently !
Xander -- it'll find plenty of food in and near the compost heap.
Quite a good chance that the compost heap is where the clutch was laid, so there may be another dozen or more of Snakey's siblings around the place.
The eggs are laid in manure, compost or something else warmish (most other UK reptiles have live young, not eggs).