I have since remade this arm to make it look a bit better. Here's how to do it if anybody is interested. First you need a new arm which has an angle ~80° and appprox 4" long bent from 10x 1mm stainless steel piping from the DIY. Make a template of the original mirror fixing and transfer it to some solid 10mm aluminium.
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File/saw to shape[frame]

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Polish with plenty of elbow grease![frame]

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Drill arm to take a 2mm pin[frame]

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The knob is from a drawer (DIY) with a 12mm diameter ball and the shaft is 8mm which fits the inside of the arm[frame]

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The new improved version.
Finished article and no more obscured vision. Adjust and then glue the shaft of the drawer knob into the arm of the mirror. The mirror fixing bracket may need some filing to get enough adjustment. I know it's not original and I could be in for a stoning from the purists, but it works and is in my opinion a safety improvement without disturbing the look of the mirror too much. Now to clean that mirror...[frame]

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Regards
Declan