Another little gremlin appeared last night on the way home where the nearside headlamp took on a distinctly pink hue. It turns out that this usually ends in total bulb failure within a short time so I sourced a new bulb from Halfords this morning for £47.99. I did phone BMW first but they didn't have any in stock and wanted over £170 each! I know there are dodgy bulbs available on ebay and other sites but these Halfords bulbs are manufactured by GE Lighting and are of perfectly good quality. They are guaranteed for 3 years with an expected lifetime of 3000 hours. The BMW bulbs are made by Osram but I fail to see how they can charge such a premium.
Fitting took about 20 minutes in the dark but I expect to be able to change the other side in 10 minutes now I know how it goes together. See comments under the photos for details.
End result is a pair of white lights again and although I didn't have much time to look at it closely, the light temperature and intensity looks very similar to the original bulb on the offside. To be honest I'm tempted to just leave the other side alone but I'll probably change it and then keep the working bulb as a spare.