PeterH:
I've got some cheap automatic light controllers that fit between the bulb and the bulb holder and switch the bulb on and off according to ambient light levels. ... I don't know what's inside, but it may even be that they already contain a SMPS
Why would they include a SMPS?
The control device is a TRIAC, the few milliamps required to control it is generally derived from a series capacitor. You also need a small resistor in series with the capacitor - 100 ohms or so - to protect the Zener (shunt regulator) from the charging current from the capacitor if your device happens to be initially connected at the moment when the mains cycle is at its peak.