Redirect to another EntryType template if no Children exist
Fixed!

So bearing in mind i'm using this method to load templates according to entry type, Clive Portman's answer got me thinking about hasDescendants.  In my _entry.html I now have something like this:

{% if entry.hasDescendants() %}
  {% include "products/_types/secType_categoryGroupOverview.html" %}
{% else %}
  {% include "products/_types/secType_productSpecification.html" %}
{% endif %}


...and it seems to work as required :)

Obviously happy to hear of any more elegant solutions.