Here's what Cassell's Dictionary of Rhyming Slang says.

"Latchico. n. [1970s-1980s] (Irish) a wastrel, a rogue. [Scot, latch, indolent, idle]."

The Scots derivation seems to be confirmed by Warrack's Scots Dictionary and Robinson's Concise Scots Dictionary, both of whom use it to mean an idler or procrastinater.

The term was in widespread use in Liverpool, where I first heard it as far back as the mid 1960s, and where it usually meant someone who was a bit of a layabout, or who couldn't be relied on or trusted. So I'm a bit puzzled about Casell's 1970s-80s ascription. Maybe it entered wider circulation about then.