ImageCatalog script - how to change layout order

Hi

I am using the ImageCatalog script to layout pages of slide scan images where the original slides are held in plastic sleeves. Due to the way the original scanning was dealt with the numbering is top to bottom right to left (Epson V700) and I would like the printed versions to keep to this.

So I would like

9       5     1

10     6     2

11     7     3

12     8     4

But currently the script gives me

1      2      3

4      5      6

7      8      9

10    11    12

in writing this post I can see that a simple method should be not to far off but my programming skills never progressed much further than BASIC from the 80's so any quick time saver would be very much appreciated.

Thanks for any help with this

Rob