Right so. That didn't work then! I got maybe a handful of charcoal from the bottom and a lot of charred wood.
So, I listened to what I'd been advised on Creative Living forum and made a nice neat stack of fresh wood on top of kindling...
... waited til it was all nicely alight and blazing...
... then tipped over the barrow again.
But, this time I'm leaving some air gaps around the edge.
Now we really are into trial and error. I'll just leave it for a bit. At least I know it's still burning under there with restricted oxygen. If this doesn't work I'll bash some holes in the wheelbarrow with a chisel and bury it totally around the edges so I have more control over the amount of air by covering holes. I might also go look for pictures of oil-drum burns and the like to get a rough idea of how much air hole is needed for what size of burn.
Still, as they say, you learn more from things that don't work!