£5, £10, £20, £50, over £100?
I don't think I'd bother going to a police station for £20 or less in just cash as I don't think most people would bother to check for that small amount. It gets a bit greyer between this and £50, purely because it would be a faff for me to take it there, but I think maybe the guilt would make me do it. Easy decision over £50.
I suspect mumsnetters will say anything should be handed in, but is anyone prepared to say differently?