Thanks for taking so long to talk through these issues which we eventually resolved. The issue turned out to be that all the DMX sources we had selected only used a low number of DMX channels, so the transmission packet only covered these circuits (eg 18 channels) before re-starting; the transmission did not include the remaining channels to total 512. The FLX-S could not see a full DMX chain of 512 channels so did not open the shorter data chain.
The tests we did using the FLX itself and our Infinity desk as alternative sources showed the FLX could read DMX in, which we initially thought was attributable to an earthing issue.
I have now tried recording a state from our Zip 12 into our Swisson and playing this back into the FLX which works.
I'd still like to be able to use a smaller desk as an input which would open up the possibilities of running sound-to-light on the FLX