I had nothing better to do, so I decided to open the throttle body. The engine has not been idling smoothly, so it was a good idea to open up the throttle body to check.
After opening it up, I found a layer of sticky carbon buildup around the idle air control (IAC) valve, the air bypass passage and the throttle. No wonder the engine was not idling smoothly. Anyway, a good clean with a toothbrush, cloth and some petrol helped remove most of the carbon.
The throttle body; throttle at the bottom, air bypass passage at the top, idle air control valve at the top right and throttle position sensor (TPS) at the bottom right.