In the last few weeks my car (MY 2010 manual gearbox with manual handbrake) has developed a clunk from the rear when reversing.

This is not the normal slack in the drivetrain type of clunk - it only happens after you've reversed about 1 or 2 metres. It's not handbrake related because it happens when you haven't used the handbrake, and it will do it repeatedly if you drive forwards, stop with footbrake, reverse a short distance, clunk, etc.

It sounds/feels like something is engaging or disengaging in the back axle/Haldex area. The whole unit was replaced by Bell engineering last year so I hope it's not that.

Any ideas?

maybe - try unplugging the haldex to disengage it and hopefully eliminate it from your analysis - dont worry about the alerts popping up they will go when you reconnect. Jules

I had a strange clunking a few weeks back at the rear two days later the rear wheel was trying to lock up turned out to be the hand brake shoe retaining pins and spring clips 1 had corroded and let go result spring clip had chewed the adjuster and the hand brake shoes and expander mechanism  2009 FL2 S now gone
2011 FL2 HSE AUTO
2012 FL2 XS AUTO
2000 Kawasaki er 5