Bingo! That did it.
And in a most unexpected way. Its replaced the embedded
"img src='plugins/data:image/png;base64,iVBORw0KGg....."
with a much shorter
"img src="cid:b709cbe248c945bc6b1....."
The full image data has been added to the end of the file in a nice neat chunk. Its really rather pleasing in an ocd kind of way.