To provide more clarity  see an abbreviated example of what I'm trying to
achieve below.

httrack <http://www.mysite.com/> '-*' '+www.somesite.com/assets*'
'+www.somesite.com/site-section' -O site-section-folder --robots=N

httrack <my-site>/ <filter out everthing> <except assets> <and this section>
<output to this folder> <ignore my robots>

Have I taken a wrong turn somewhere ? 

Thanks,

Mark.