Panels "Page not found" when using required wildcard argument
The problem was that my taxonomy terms had dashes, e.g. "Early skills for non-training grade doctors".  When used in conjunction with views contextual filters and the option to "Transform spaces to dashes in URL", it doesn't match properly, so no value for the context is provided.  This is related to the following views bug - https://www.drupal.org/node/672606 (no solution as yet for panels but there is a solution if you're just using a view page in comment #16).  My solution was to remove all dashes from my term names.  Not ideal but fixes the problem.

Here is how I set things up to pull a taxonomy term name contextual filter into a View Pane, in case it helps someone.  If anyone knows a better way to go about this then please let me know and I will update this answer:

View Setup - Contextual Filter





View Setup - Argument Input

This part is counter intuitive, you have to choose "No argument input".  Providing a context from panels will prevent the default filter value from being used.  Thanks to the following comment for helping me figure this out: Views contextual filters taxonomy name





Panel Setup

N.b you have to use a panel page here (structure > pages), so that you can make use of the wildcard feature.  I tried this by panelizing a node initially but couldn't get it to work.

Basic settings:



Argument settings: