Ah Ha, We sussed it!
Basically, when a line-break is used in a post the archive view will display everything above it including any inserted image plus the ‘read more’ link. When its not used you will only see the title, a text excerpt and the ‘read more’ link.
Actually, this is quite handy to know because then you can actually choose the default view in the archive listing simply by using one method or the other.
“Who says falling over has to be painful” ๐