Although Tinhead Ned was good at escaping prisons, he didn't run very fast, and was caught and executed.
This, as the Eksians would put it, makes for a good ballad. In fact, Rincewind was able to escape mainly because of their intention to make a better ballad - they encouraged him to escape, and gave him a headstart.
In Roundworld, "Tinhead Ned" is slang for "nail" (item of hardware). But the character seems to be derived from the Australian bush ranger, folk hero and outlaw Ned Kelly, who was hanged in Melbourne Gaol in 1880.