So the feedback looper didn't work - it had power, switch turned the led on and off but not outputting any sound.
Thought it was probably a dodgy connection on the switch but they were fine, all connections and solder joints were ok too.
Turns out it was the return jack. The tip connection wire protruded through the hole more than I thought and was touching the case. Sorted that and now it's working. Phew!



