Thanks for that solution Jofm5 I down loaded text pad and played with macros but frankly it's actually quicker to import the files into acrobat in 3 lots ie 1 to 9, 10 to 99 and 100 upward. I was hoping someone might know of a box to tick in acrobat to sort the problem as I couldn't find one.
I still find it slightly odd that it should re-order things, after all, all I'm doing is clicking the first file to select it, scrolling down to the last one and shift clicking to select the lot, right click to copy and then right clicking again to paste it into the "add files" box in adobe.