Sorted - found a couple of things buzzing!
First was the air vent duct, the metal push-fit washers had worked themselves loose, so put a strip of trim tape over the tab/hole to make a tight fit then refitted the washers...
…which uncovered another buzz, which I discovered was one of the wiring harnesses going into the digital dash. Used some thick squishy foam to pack the area out, and quiet as a mouse...
…which uncovered another buzz, which was the wiring to the dash cam rattling against the back of the glovebox, so I cut the positive wire shorter, put some sticky back foam tape around the piggyback connector (that was loose), tidied up the negative cable, put some foam sticky tape where it was close to touching parts of the car, added some sticky back cable-tie mounting plates to dress the wires, et voila, quiet!
Thanks again for your
help @LeylandVCDS as I wouldn't have thought to look at the vent for the noise!
On a different note, I also wanted to fit the Osram Dynamic indicators, however after Amazon sending Audi A3 lights twice, then a counterfeit set of random lights in the Leon box, followed by a third set of Audi A3 lights, I’ve given up for the time being!