They're both right y'know. :)
You MUST by law inform the dog warden asap. Many don't have a 24 hour service but the council should have an emergency contact number which will advise on where you should take the pooch IF you so decide to do so, though I'm glad that you won't. :) Lucky pooch.
You have the right in law to keep hold of him unless the council have good grounds to believe that the dog's welfare is at risk. In doing so you must agree to keep him for at least a month and not hand him to anyone else bar the owner, which you should make reasonable efforts to locate.
If they get stupid about this because they don't know the law then give m a shout and I'll ask an RSPCA lawyer contact I know to put them straight, he likes doing that! (Been there, done that, had the argument with feck off great 17 stone shaven headed dog warden who was a bouncer in his spare time and who had studs in the back of his neck... I'm 5 foot 3 and at the time weighed about 8 stone dripping wet!).
If you want to cover your ass in the absence of anyone to report to tonight you can ask the police to record him as a found "item". They now only deal with dangerous dogs and not strays and have no obligation to record a found dog but will do if you press them to, you never know, the owner may be unaware of this and may have by now reported his loss to the police (who will have told the owner that they don't deal with strays unless dangerous etc etc but who may, if they are on the ball and kindly, have recorded the loss anyway).
Please check DogLost for a missing thread tonight and tomorrow/thereafter and post finding him on there - ask their advice on how to identify the owner as genuine if he contacts you to claim his dog, I'm not quite sure what they suggest tbh.
And wrt to posters etc, you may be wise to just say something to the effect of "Found, young black and tan dog" if, for example, he's a German Shepherd, or "found, young yellow dog" for a yellow Lab etc, just to be on the safe side and deter dodgy people.
I wouldn't post a pic on Twitter if I were you, just in case - a rough, vagueish description might be safer.
I'd have him on my bed too, btw, but I'm a soft touch like that! I'd rather that than a miserable pup crying half the night. Guess my neighbours would too! :o
And of course, albeit that he's a pup, you don't know him so please don't leave young DC with him... a child being over enthusiastic, falling on him or whatever could result in tears all round.
If all else fails, parcel him up and send him to me, at.... :o :o :o