====================== Light oxidation can be removed by rubbing with a cloth soaked in patination oil. Heavier deposits need pre-cleaning with wire wool. Try 'Brillo pads' - they clean more gently than plain wire wool. Try not to scratch the lead too much.
How long the effect will last depends on local air pollution so you will have to repeat the treatment from time to time. Since your flashing isn't really decorative it probably isn't worth doing unless you're very 'house proud'.
Cic.
Cic.