Over 50mph….i hear a fluttering noise which gets louder as speed exceeds 50mph

Also

When turning fairly hard right - e.g. on mini roundabouts - i hear a soft knocking noise

Propshaft replaced by main dealer <12 months and <10k miles ago

Wheel bearing?

Sorry no idea about the 'fluttering' noise, but as for the knocking noise, first step for me would be to get under the car, use a pry bar to try to move components, anti roll bar, drop links, suspensions arms etc.
I doubt that the wheel bearing is the cause of the knocking, when a bearing starts to fail, the usual noise is more like a 'droning' noise. Hope this helps

Forgot to add, also check for play in the drive shaft joints, check all these on the off side first, as when driving round a tight roundabout the nearside components will be under more load and less likely to 'knock' there are numerous possible causes.