@schnapps - Thanks for the info on that hidden menu.
Alas most of the wireless options didn't seem to do anything. Power did seem to cut the power of the transmitter a little for a while but I think it reverted to whatever the original setting was after a bit (I'm guessing after it had been in standby)
It seems a strange behaviour on Roku's part to set a WiFi transmitter to continually hunt down & out power your router.
I guess I need to live with it or pick up one of the standard Roku IR remotes so that I can disable wireless on the box. It's a shame as with the Ethernet I'm getting the full 720p on the Now TV channels & the 4K HDR on Netflix.