Thanks for all of the tips, I have actually made some progress and it is now much better than it was (although still not perfect). What I had to do was in the Legacy body HUD there is a tab to select the neck tone that is the closest match to the skin (numbered from 1 - 32), then you copy the UUID of the corresponding numbered neck fix texture file and apply this to the head in the Catwa HUD and use the blend slider to blend it in.
Re the neck size there are several options in the Catwa HUD (Jake, SL, Sink, TMP, Adam, Giani), none of which seems to relate to Legacy bodies and having tried them all the only option that looks anything like right is with this section switched to "off" so I can only assume that is the correct choice.
As it is now there are still some imperfections when zoomed in close but it doesn't look too bad from a distance so I guess I'll have to live with that for the time being. Thanks again.