Thanks. I’ve got it working. After taking it all apart with no luck, trying a different sd card and then working sdcards from some some of my other zeros which are ok, process of elimination led me to the answer. I had copied the whole downloaded keybow firmware zip contents to my sdcard rather than just the contents of the sdcard folder. Doh! It’s now working great! Must read instructions more carefully in future… I now also know that if the sdcard contents are wrong the zero shows no signs of life at all. I thought the green light would come on to show it had power. Might be worth drawing people’s attention to that in instructions for this in case anyone else is as dozy as me - manually copying files to the sd card has more chance of ballsing it up like I did. I’ve never bumped into this before despite have several other pi zero projects because they all involve writing an image to the card.
Anyway, here’s 3.1415926536 typed on my Keybow!