Most vets will keep a stray or the animal rescue services can normally help and if it's a dangerous dog I would think you could call the police. LOL I dare say the "cheeky little sod" on the phone didn't decide the hours ewww I would hate to take "those" types of calls, normally they have a blanket response as they get a lot of irate, abusive etc calls (no I am not saying yours was lol).
We get the same response of hold the dog until morning if the ranger is very busy or they ask if you can just for practicality, I'm surprised they said to let it go, it is worse on weekends.
I had a quick look at the council website here and couldn't find anything about the hours set out for availability, but I would think it is an individual council decision based on need and funds, maybe ring the council office and or have a chat to the ranger or look up your local council animal info on their website
ALL kittens are devil spawn wrapped in cuteness!