W220 2002. Headlamp failed suddenly whilst on daylight running. Changed the bulb but still u/s. Seen plenty of questions online about electronic fuses but no answers!

 

"Electronic fuse" is not really accurate.
Many of the electrical circuits on the car are monitored by one of the many computers that run the systems. The computer will have a programmed range of current draw that is acceptable. If the current draw on that particular circuit is outside the acceptable range (either too high or too low), the computer will cut off the current flow. It will check the circuit periodically (quite often), and if the draw becomes within range, normal current flow will resume. There is nothing that you can do to "reset" the circuit - all you can do is to check the circuit and try to determine if the problem is an open circuit (broken wire, faulty connection, faulty ground, blown bulb, etc., etc.) or a shorted circuit.
In this case, you can check the resistance to ground at the lamp connector, then check the resistance through the bulb filaments.