Thanks David,

That's unnecessary, using $device (.....) as the context is fine.

'Context' is the magic word that made the penny drop. This:

$subdevice = $device->find('/subdevice', $device);
[download]

Does just what I need.

I was only looking at stuff in the debugger to try to understand the difference between nodesets and node lists. It wasn't helpful as both are far too large to read.
All I could really gleen was whether my code was giving vaguely sensible results.

Thanks a mill
Nonk