£1000 you have to do the decent thing and hand it in. A fiver might be different. On two occasions when I have handed money in there have been no thanks from the losers
Not QUITE the same thing, but I found a very expensive camera a few years ago, with some rolls of film and some cash. Took it to the nearest police station and 6 weeks later my ex husband received a letter of thanks and £50.
I'd hand it in. Despite the fact that £1k would be useful, it could be more than useful to the person who lost it.