Thanks for that SD.
I tried this...
(The second line only - tildes(~) omitted)
$posts['POST_DEFAULT_BODY'] = $html->truncate($posts['POST_BODY'],$feed_include_body);
$posts['POST_DEFAULT_BODY'] = str_replace("%%~GRAEMLIN_URL~%%",'graemlins/default',$posts['POST_DEFAULT_BODY']);
...however, it doesn't seem to make any difference at all.
I also tried "images/graemlins/default" and GRAEMLIN_URL without the %%
Any thoughts as to what could be happening?