thanks for the advice Jon. I've tried dropping the font size and
turning on the autoscale - that seems to make them appear but only at
fairly illegible font sizes!

I think you might be right with the squeezing theory - because it's a
multi-series side by side stack there are actually lots of columns
displayed but only some have an x-label. I was expecting XL to make
use of the 'extra space' but evidently it does not!

Thanks again
Rob