OK, well one of the great things about blogging is that it helps clarify your thoughts and so it has proved. Further fiddling with the spreadsheet and general thought has made me realise that the Kronecker Delta $\delta_{mn}$ applies to integer values of $m$ and $n$ and that neither $\int_{-\pi}^\pi sin(x)sin(2.5x)$ nor $\int_{-\pi}^\pi sin(x)sin(9.5x)$ are zero – as the graphs actually suggest anyway….