OK so weird... I have five tracks, checked them all and they're all labelled correctly and in lower case, 1 channel, 22,050 and 16bit. I think I was CMD+C and CMD+V previously as I usually do with any file, but I wasn't doing that on purpose and so tried again a couple of times and swapped the wav files around just in case. Still wouldn't work.
Then I copied the files onto a USB, transferred from that USB to the SD card via my friends old PC rather than my Mac, and now it works! So appears to be some issue happening specifically on the Mac device (thought my mac is 7 years old also and the only machine I've used on the saber since I got it until this week).
So hurrah it is now working, thanks so much Erv! But so strange that it won't work on one machine but does on the other.... Is there anything else I could check that may interfere when using the Mac? 10.13.6