In the UK many S3s (esp. the later ones) had a different mirror arm unit that mounted to the sides of the top hinges using the two existing door screws. This was made by Britax and is easily found in most of the UK specialist parts catalogues - e.g. Craddocks, Paddock etc.

It looks a lot neater in my (modest) opinion than the later Defender units which sit on top of the hinges. Really comes down to how close to original you want your Series 3 to look. If it doesn't matter, go for the Defender unit.

Cheers,