Finally got my - and Liz's - act together to register for Mexico City's equivalent of Boris Bikes: "Ecobici". They're lighter but still very solid, kept in good nick, three gears, easier than BBs to take out of the docking stations (of which there are 275 dotted around the city centre) - and at 400 pesos (£20) for a year's use, dirt cheap.
Introduced in 2010 by Mexico City's then Mayor, Marcelo Ebrard (Las Bicicletas de Marcelo?), the scheme has been a huge success. Everyone uses them and I'm surprised the city government can keep up with demand. My first ride was all of 800 metres from one docking station to another, but it felt good to be on a bike again.