Morethan have just launched Drivetime - www.morethandrivetime.com - a service that knocks 40% off their standard quote for under 25s as long as the car isn't driven between 23:00 and 06:00.
They install a GPS thingummy to show when your engine is started and stopped - keep the engine off between those hours and you're saving. Apparently if you're driving at 23:02 then you're breaking the rules but you're still insured - you just pay a £25 charge each time you drive between them hours.
From my POV (age 24) this seems ace - I start work at 08:00 so I'm rarely larking about at 23:00 so I reckon I'll go for it (although their site fell over with all the interest, ha!) depending on their standard quote price. It also means that I can charge the parents next time I am a drunk tank taxi for them - sweet