Hi Gavin,
New items are manually added throughout the year. When rolling out a price rise all items are exported to a text file, the Selling Price is updated, and the existing products in MYOB are updated via the import process.
The "Item Number' and " Selling Price" are matched during the import process, but the 'No. Items/Sell Unit' is not involved.
I have been having this problem for many years, and as mentioned earlier, it does not affect items that were 'fixed' last time the scrpit was run, only products added manually since the last time the script was run.