Thanks both for the suggestions and tips. Didn't want to sound like a total moron, asking about what should be a relatively simple task, but wasn't sure if there were any hidden (read BMW) issues...
Will be replacing both HID bulbs while the expansion tank is out...wish me luck with the fiddly torxs screws at the bottom of the light covers...

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08/06 build 4.4i Stirling Grey over black