Person checks into hotel with dog, then checks out and leaves dog behind.
When called (on mobile) owner says "I don't want the dog; get rid of it or keep it yourself".
Hotel owner calls Dog Warden; not interested, the dog is NOT a stray and has been "gifted" to owner of hotel. They won't take it into their care and if it's released then the hotel owner can be prosecuted for abandoning the dog.
Hotel owner and dog warden try to contact owner but they don't answer telephone.
Couple of weeks later owner reappears and asks for dog back.
Turns out that they have been on holiday and were not prepared to pay for kennels and no friends would look after the dog. Didn't want to risk losing dog by taking to RSPCA or Dogs Trust and having it rehomed. It appears that legally they can do this, then ask for dog to be returned and pay nothing for care of dog!

The way the law is written if a person finds a stray and dog warden takes dog, after 7 days it can be rehomed and they can "transfer" ownership. If dog is kept by finder then after 28 days they can become the new "owner" but legal ownership still reminds with previous owner (with proof of ownership). In second scenario the person who finds dog has a legal requirement to look after dog, pay for food and vets fees. However the "legal owner" is not required to re-imburse any of these expenses incurred by the finder.

Legally it would appear the the owner saying "I don't want the dog; get rid of it or keep it yourself" transfers ownership but there is no paper trail to confirm the new owner's status.
Now appears that the hotel owner can be prosecuted if they keep the dog as the dog is classed as "chattels".

Has anyone got any advice on this for the hotel owner/good samaritan?

That is outrageous and I'm not up on the law, but can they not claim abandonment? Having been a volunteer for breed rescue I know quite a few owners that hand their dogs in because they can't afford/ don't want to pay for kennels, but obviously they have to sign the dog over. If they do have to hand the dog back, the owner of the hotel should be able to claim costs.

Could the hotel not bill the owner? Say he gave the dog the penthouse suite.

Dogs can be taken to cover the cost of the debt.