TKF - I believe that no matter what sort of tightening you do, the headset will still shift from time to time; either that, or it will be incredibly tight and extremely uncomfortable.
You may wish to purchase one of those psuedo flying helmet things, which may people use for aeros to handle the problem that you've identified. I don't know where you would purchase these in the UK, but here are two sources in the USA:
http://store.yahoo.com/perroneleathe...tsgoggles.html
http://www.flightsuits.com/open.html