The flicker is caused because the amount of power needed to run your HID's at first is very large and your car needs to power up in order to start them (HID's work by 'igniting' a gas, rather than by causing a filament to glow as it would in a halogen bulb), if you look at your lights under your bonnet you should see two boxes, these are ballast boxes and they work like capacitors (I think), storing power to feed to the HID bulb, this is why your lights flicker, or so I have been told, on startup, because they drain so much power, the ballast can't keep up with the initial drain.
Happened on my last car all the time, and it happens on all others too.
Bulbs are from memory around 40-90 pounds each depending upon type etc. etc.
Tom