Sometimes, my mind works on unsolved problems during sleep, such as those baffling slives. If a really ocker Aussie talks about slives, they mean slaves, as in, "Are you bringing your slives to the barbie this arvo, mate? And don't forget the snaggers, the six-pack and the mozzie spray. Gonna be bonza. There'll be a footy to kick around too."
I was a bit mystified about the shirt with long slives, but since they're already bringing a Bridget to do all the dirty work for them so they can keep their breeches spotless, perhaps the slives are mentioned in conjunction with the shirt, and have to be long, so they can keep the rider in their white breeches well off the ground until the horse is available to take over this task, further reducing the risk to the spotlessness of the breeches and general attire before the actual event.
SueC is time travelling.