Anyone got any links to videos on how it’s done? It looks to me like I have to basically reinstall the entire cable?

Yep, new inner cable and possibly cable housing needed, if you have enough cable + housing length using the existing thumb KGSl remote then you may get away with shortening the existing housing by an inch and cutting an inch from the clamped cable end so that you can reuse it/pass it through the housing but for the sake of a few £ i’d personally just change the cable n’ housing.

I dunno about videos as it’s a pretty simple swap – maybe ks have them on their site?, if you can change a gear cable then the southpaw will be a doddle.