Herman,
Thanks - but that wasn't it. I tried cycling the filter from "All", to "Group" and to the "VC", but the APs would not show up.
Turns out that somehow Central lost track of my VC IP Address (see attached). I suspect this may have happened as I was playing around with forcing a different Preferred Master. At one point I had two different APs set as Preferred Master and they both came up as the VC separately - this must have confused Central.
Once I put the IP address back in "Wireless Management", "System", "Virtual Controller", "Edit IP Address" all was well again.
Thanks for helping to sort it out.