Found cat

The law states that you must first report it to the police.(I know right?).
Then you can take it to one of the refuges and they are obliged by law to hold on to the animal for two weeks while making an effort to find the animals owner.
If after that two weeks the owner has not been found and the animal is deemed healthy enough it may then be allowed out for adoption. The owner/adopter will then pay all cost of incurred of those two weeks.
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So that's the official line, first step report it to the police.