Hello PhilGlennie,
Welcome to the HP Forums!
I understand your Officejet 6700 will not print on both sides using the long side feature. I will do my best to assist you! I would recommend following this entire HP document on Printing on Both Sides of the Paper (Duplexing). Try changing the "Print settings" on your computer.
Please post your results, as I will be looking forward to hearing from you.
Have a great night! :smileyhappy: