To cut down on glare? Do they exist, do they work and if so are they legal? And yes, I realise it seems semi riduclous but I have heard of such a thing.

Yes they exist, but they dont exactly cut down on glare as you think of it, they're yellow and they prevent your low light vision receptors from being bleached by the white light of oncoming traffic, so that after a blinding your vision recovers more quickle, but it wont stop the blinding and I suspect if you used any to stop the blinding you'd be 'ad for being a muppet. Unfortunately with the evolution of HIDs and poorly adjusted headlights the need for wearing shades at night has increased.