@David-TP Thanks for your reply. I've spent quite a bit of time on this. The problem seems to be with the channels the decos pick. 2.4Ghz channels are quite congested here - inc some hidden SSID networks. The network optimiser consistently makes 'interesting' channel choices - feels like it isn't looking at all networks and so picks badly. I suspect the firmware update simply forced a change of channel - it's now resolutely stuck on a poor choice. 

  @LTP290 Also for 5Ghz, it seems to pick the lowest channel rather than some much freer, higher band channels?