Thanks guys, sorry for not uploading the video's earlier, I was at work, and all I really had access to was creating that animation.

Anyways, here's the videos. The first link here is powering the printer, and then a single G28 followed by a G29. The second video is the same cold start with G28 then a secondary G28 followed by a G29.

Video 1: https://www.youtube.com/watch?v=HPX7U34GF2I
Video 2: https://www.youtube.com/watch?v=D3AJEj16r1U

Interestingly, as long as Repeater Host is kept running through out, any secondary G28 command will behave like it does in the second video. That is to say that I can disconnect, power cycle and reconnect the printer, and as long as I've kept RH open, the second or any subsequent G28 commands will zero the axis and then return to centre.

I'm running Dacb's firmware mod as at the time it had everything I needed in there, and pretty much configured already for me. I've included my Marlin_main.cpp as well

Marlin_main.cpp