there is a legal requiremnt to have them on HIDs, that's the reason aftermarket ones are advertised for off road use only.
Steve
Not that i've spotted. :-? Most are supposedly 'e' marked and 'iso' approved also 
Stevo, have you got some pics of your motor in the gallery? Looks to be just like ours and i've never seen another one before

Sorry guys, your adding apples and pears and coming up with oranges as an answer......
See the WIP in test on headlamps. Dont confuse real HID with aftermarket 'xenon' H1, H4, H7 replacement lamps for projector light units.
Realdeal HID's are fitted with a lamp washer system by the manufacturer as a legal requirement. Washers become a grey area in the second hand fitment side of things if you have projector lamps with a 4000k, 6500k or whatever lamp fitted.
The HID lamps cost about £60 or so each but the prices are dropping. Thats for the lamp filament, not the complete unit. The H4 or whatever xenon filled lamps are whatever the market can stand, around a tenner each or so but again, prices are falling.
All must be 'E' market to be legal for UK on road use, as does every lamp on the vehicle. Its one of the ways that there is to deal with a chaved up whatever with blue/green/whatever colour side lights to the front.
HtH
B