Thanks to everyone for their interest in little Sonic. Just a quick update (cuz I'm shattered!)
He made it through the night (hooray!) I checked on him around 2am and he was having a drink which was such a confidence boost. Well pleased! This morning he was snuggled up in the paper. I disturbed him to clean out the cage and feed him, then went to work.
Momentary panic when I got home and he was lying outside of the paper... but I got him out and put him on the weighing scales and he was walking around fine which is brilliant!
His weight? A tiny 278g... this time of year, for a hedgehog to have any chance of surviving hibernation it should be at least 600g... he's unfortunately gonna be in captivity now until the spring, when he can be released. At the very most, when he reaches 600g, he'll be put in an enclosed foster garden and fed every night. Poor guy...
Anyway, I've uploaded a photo to the album. Feel free to check him out :-)