Just had a brain wave and found the problem!
The amount of products order in the table orderdetails was greater than those on display. Having looked at the code on the page, it was showing only five items when seven had been ordered, so the repeat region on the page wasn't displaying all products.
Looking at the repeat region code for the recordset though, it seems to have a repeat of 5 which then wraps to another page should the amount of products in the cart be greater than 5... so it looks to be creating a paging effect, but that doesn't show on the admin page.
$maxRows_WADAorderdetails = 5;
$pageNum_WADAorderdetails = 0;
Is this code correct?
I've overcome my problem by changing 5 to 500. If somebody has the time to order 500 individual products in one go, then they have issues!