@stujomiles 

Hi, 
Deco provides 2.4G and 5G at the same time, if you have a device that supports both Wi-Fi bands, it should be fine to connect the device to the Deco. If the device only supports 2.4G or 5G, you can also switch off one Wi-Fi band on the Deco app to have a try.

For the speed problem, how many speeds do you have when connected to the virgin hub by an ethernet cable directly? How many speeds do you have when connecting to the main Deco by an Ethernet cable?
When connecting a device to the main Deco's Wi-Fi network, how many speeds do you have on your device?
 

By the way, whether the main Deco is the two slave Deco's source Deco? On your Deco app, you can click on the Internet icon and click on a slave Deco to see this source Deco of the slave Deco.

@stujomiles 

On your Deco app, you can click on the Internet icon and click on a slave Deco to see this source Deco of the slave Deco. And only you can see the signal strength from the source Deco, with this information, you can place your Deco in a suitable location.

Which mode have you configured on the Deco? Router or AP?

If they are working in the Router mode, you can set up your VirginHub 3.0 works in the modem router mode, and then configure the Deco works in the AP mode, and check if the speed will be better.

If Sky Q only operates on 2.4G, you can enable the guest network and turn off the 5G on the guest network, then connect the sky Q to the 2.4G guest network.