Hi Ewan!
I was in this exact-same position when commuting in +5°F to -12°F weather in my uni days.
I went to the local plastics shop and got a sheet of 2mm polycarbonate, warmed it in the overn, then took it out and bent it 'round a coffee tin till it cooled. Drilled a hole in each upper corner, and attached a bungee cord. The cord went 'round the helmet and was held in place by the shape of the rear vents. It worked great but for two things:
1) It hit my nose ("dose" when the visor was against it, which didn't help breathing much).
2) As a consequence of 1) it steamed up from my breath.
Giving up on that approach, I found happiness in a downhill ski-racer's goggle-and-mask set, made by Scott and designed to protect the faces of slalom skiers who often hit the fiberglas poles that serve as gates. It even came with different-colored lenses (clear, smoke grey, and amber) and had changeable filters in the vented portion of the mouth shield and worked great. I haven't found them listed for sale in recent years, but a nearly identical product is offered to off-road motorcyclists.
Mine didn't flip up, but it was easy to simply grab the face guard and pull it down and let it hang by the strap ('round my neck, rather than head) when I didn't want/need it, but wanted it ready to hand "justincase".
Except for being white in color, mine looked exactly like this:
http://www.ebay.com/itm/Vintage-MX-Scott-face-mask-w-goggles-CR-RM-YZ-KTM-BMX-/190564907921?item=190564907921&vxp=mtrFor motorcyclists:
http://www.totalmotorcycle.com.au/category/15_offroad_goggles/subs/20_scott_goggles/products/925_scott_83x_safari_facemask_goggleCurrent offering from the snow-machine world:
http://www.firstplaceparts.com/Scott-Safari-83X-Facemask-Goggles.htmlVery similar mask-and-goggle sets are available to paintball enthusiasts and might be worth looking into. An example:
http://www.amazon.com/Scott-Vectra-Airsoft-Paintball-Accessory/dp/B004MAWU20I'm shocked at the prices, having paid USD$18 back in the day (a "day" longer ago than I remembered, till I saw how prices have increased) for my goggle-mask combo. It really did the trick in heavy rain, severe cold, and snow. I hope something in here helps.
Best,
Dan. (...whose biking-related efforts have made him a poster-child for alternative fashions)