Thanks Jason, but I don't have eCart 5 - I'm using 4.5.2
Subsequently there's no Local Checkout Response - only Local Checkout...
The selection are different...
Quick Update:
I did get a display of the error message...
Line item 1 is invalid
is what it read... I read through 100's of pages and found that that usually meant the "title" has incorrect characters etc... So I hard coded an appropriate title and got the same result...