How long does a Postal Check (postal order) take to clear?

Uncrossed postal orders can be turned directly into cash and thus definitely take no time to clear.

Crossed postal orders have to be paid into a bank account, and so it's possible the bank would take some time to clear them. However they can't be rejected because of insufficient funds, as the sender already paid for it.

I don't have any personal experience of them and in particular I'm not quite sure what happens if you pay in a postal order that either already has been or is subsequently reported lost or stolen. The Post Office FAQ is my main source of information.