Buddypress/wordpress display members alphabetically by surname

Hi, I hope I can find someone to help me.
I am using buddypress on my site to manage members. In the loop, I set up alphabetical order by surname as per @mike_cowobo suggestion here:

The only problem I have here is that It is using the “display name” rather than the surname to order by, therefore some members get lost in the order.
Would someone be able to let me know how to alter this coding slightly so it uses the last name instead of the display one?

Just so you know, I have already tried changing display_name to last_name but it doesn’t work.