There are no router settings you can make on the SH2 that will have any effect.
Does the hub Wifi stop working when you connect just the switch with no APs connected?
Do the other Ethernet ports on the hub stop working as well as the WiFi?
Thanks for all the advice everyone!
I've tried it all and then some -- ethernet etc. Seems it's just the wifi that is affected.
In the end I have resolved this by getting my own router. Cost £60 for a good one. That's £60 I did not want to have to spend, and seems a real shame that BT don't provide better information or support on how these things can be set up on their equipment, but at least now I have exactly what I need and it works a treat.
I guess I'm curious as to what "good" router you managed to get for £60? Also how were you testing that the WiFi on the SH2 was down? Remember that 5GHz WiFi on DFS channels can drop for up to 10 minutes when you change any settings - while it checks the selected channels are clear!