Hi Callum,
I have discussed this issue with Support as I have run into it before and thought I had it working but I realised that it worked when Creating but not when editing.
I have sent you this privately but for other benefit.... here is the fix from Support...
" Popup window inside a popup window is a problem with auto-refresh. If you make the first window (order header) modeless and add OrderLines object into the auto-refresh options of the OrderHeader form it will fix both problems."
The referred to "both problems" means..
a) Updating a Total in the Parent (SUM of Child Record values)
b) Dialog asking to save the changes to the Parent