How do you know the switch is fine?

WITH THE POWER OFF: At the light, check continuity between the switch wires (test between the block of reds, and the black next to the brown). You should get continuity with the switch on, and no continuity with the switch off. If you don't get any continuity, then short the wires at the switch (can put them both in the same switch terminal) and retry. If you start getting continuity, replace the switch. If still nothing, you have a broken wire between the rose and the switch.

If you have a broken wire between rose and switch, there's probably a screw through it somewhere - check for pictures mounted above the switch. Replacing cables is a right pain - if you have attic access follow the wire up there and check for damage. If you're super lucky the fault is in the attic and you can replace a section (with an attic-accessible junction box) without damaging walls.