> I gave you no help at all. :-)
>

Giving help <> Providing complete solution.

We didn't quite get to the bottom of it, but that is more because of
me not getting to the bottom of why such an index would be more
useful. Your statements got my brain thinking again, especially
suggesting that the difference in performance may be other factors. In
any case, it works fast enough now so on to optimising something else.

Adam