Okay, so I couldn't stay up until 2am this morning as I''m currently in Greece.
But when I woke up this morning at 7.30 (5.30 UK) I was able to book JNB - LHR in CW on BA.com
I did check first and Avios was showing the block for 2 Jan as having availability but could get further as it then said nothing was available (I thought maybe it was just a bit buggy). I've just checked again now and 2 Jan block shows nothing which makes sense as I've booked earlier on BA.com perhaps the inventory got updated.

But I just spoke to Avios.com, as a trial to see how long it would take to make a booking. Trying to determine whether or not to make the calls back to back or to have the cancellation discussion with BA and have Avios.com on hold.

I mentioned to Avios.com that if someone was to cancel their seats would they go back immediately into inventory so that I could book. I was told BA seats wouldn't go to Avios.com, only Avios.com seats. They have different inventories apparently.

Does anyone have more info on this?
Also, which is a quiet time to call so that between cancellation and booking someone else doesn't take the seats?