So I managed to enter Cartagena de Indias OK without an onward ticket.
In fact, at the eleventh hour in Cancún (well, about 9pm on Monday) I met an Argentine who showed me how to make a free flight reservation just to calm my nerves. But then they didn't ask me anything when I got to Cartagena, despite what I would call a shaky performance at immigration. ( I always get so nervous).
They did, however, thouroughly search my bag at customs - but only because when I pressed the button that randomly shows a red or a green light*, it came up red. Obviously I used up my good luck at immigration. No complaints there.
*The same thing had happened coming through Tijuana on October 1 - to both me and Sam who I was travelling with. (Very bad luck apparently). The guy at the desk said how annoyed he was with this new system of button-pressing. It meant he had to search good fellas like Sam and me and left him unable to discriminate, he explained, nodding towards a bandanna-wearing Mexican walking past us. In the end he gave our bags only the most perfunctory search immaginable. I could have been carrying a small nuclear detonator and 5kg of cocaine and he would not have noticed.
The customs officer here in Cartagena was sincere but sympathetic. I got a laugh for my hip-flask of Tequila and kid's cricket bat and was on my way after about 7 minutes.