Right, if you have power to the headlight, and the bulb will not light, then the problemn is in the earth (ground in US lingo) side of the circuit. Check resistance from the negative (ground) terminal in the headlight bulb connector to ground--should be very close to zero. If you read resistance, check the chassis stud below the hood hinge. Take off the terminals, clean off crud&corrosion, then check for continuity to the headlight connector.

Most likely culprit is corrosion on the terminal, second most likely is broken wire. If it's a broken wire, it's usually easier to run a new wire rather than find and repair the break buried in the wiring harness.

Wish you well with it.