Hi there John,
ChowrenToys and I actually took this one offiline a few weeks back and resolved this one together. But Thanks very much for replying!
For reference, the issue here was an internal change on the SalesPrice field from a Decimal field, to a Money field which was causing some issues while saving.
The fix for this is to remove the field (and any references to it within actions) from the screen, and simply re-add it, which resolved the issue.
Thanks again for the reply John :)
-Ben