I explored this a bit several months ago. The problem is that you have to buy a HID ballast to drive the lamps. You quickly find out that that is where the expense of HID lights comes from. The ballasts alone seem to all cost at least $125-150. I decided that it defeated my whole purpose to get involved with an expensive, hard-to-obtain part. I suppose you could build a HID for under $200. Unless you really know what you are doing it seems like an expensive gamble to lay out those big bucks for one part that you will not be able to return.
When it gets right down to it the main benefit of HID is that you get brighter light and longer run time with a lower power draw which means you can use a smaller battery. I decided that I could come very close to HID with a Phillips energy saver bulb MR16 halogen lamp producing 35W equivalent light for 20 draw and get over 2 hours of run time from a not-too-heavy 5ah sealed lead acid battery. On a second bike I have duplicated HID light with a 35W Solux high color temperature ("natural light"/very white) MR16. I use a slightly heavier 7ah sla with this lamp. A 5ah provides just under 1 hr of run time, but I prefer not to draw the battery that hard.
I do remember coming across some sites that discussed building HID lights for scuba diving and spelunking.
Are you in Glasgow, Scotland?
FWIW,
Raymond
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