Well I think I have found the reason for the dual feed foot not feeding my sandwich of fabrics - I hadn't properly attached the top lever.
I found this by reattaching the foot and this time ensuring I watched where I attached the top lever. I then tried the same sandwich & et voila it fed perfectly!
I had used the dual feed foot NOT properly attached for quite a few projects using seams, bizarre that it seemed to work fine all that time.
