Thanks Hildener, its not just the distance, there are three load bearing walls between SH2 and device. I've used the BT signal strength app, which indicates that the signal drops off after 4.5m and two walls, leaving one more wall and 4.0 m for the Complete WiFi disc signal to reach the Echo Dot in the location it has to be. If the two emitters are of similar power it should be ok, but hardwiring via Flex600 would eliminate the uncertainty and presumably provide better performance. I'm trying to avoid spending £100 on the disc only to find it won't work.
