It is not as difficult to get inside the Imperial 70 as it looks, although I do think that this machine was a retrograde step backwards from the Imperial 66.  Hook up the drawband and remove the carriage.  Remove the back plate, which is simply sprung into position.  Place the back of the machine over the table edge and put your hands under the back plate.  Flex it outwards and it will spring off.  Remove the two small screws from the top of the cast cover above the back plate.  Remove the end of the drawband from the hook on this casting and gently lead it up to the escapement wheel and leave it there.  Turn to the back of the machine and loosen the two small screws that sit in slots in the cast cover.  The cover should now lift off upwards.  Now you need to remove the tabulator column from the back of the machine.  Two screws towards the bottom, and two short 'screw-spacers' (hexagonal) at the top.  The tab. column will now lift clear.  Now you need to remove the top plate to expose the type unit.  The ribbon drive mechanism is on the underside of the top plate.  Two long 'screw-spacers' towards the back (that the cast cover sat on), then two larger screws towards the front.  Remove these and the top plate will lift straight off.  As they glibly used to say in car repair manuals, 'reassembly is the reverse of the dismantling process' !  Good Luck 

In my opinion, the Imperial 66 was the ultimate development of the 'modular' concept and the 70 was a retrograde step.  It is about 90% if the 66, and probably 90% as good !  I'm speaking in an engineering sense of course.  Since the type unit is nearly identical, I should think that the typing 'feel' is very similar so a pleasant enough machine to use.  The Imperial 80 was an even more retrograde step, and then the factory closed.  The Imperial 90 is in fact an Adler Universal 390 with an Imperial badge and actually a very good typewriter !

The result thanks to advice from thetypewriterman was a complete success. Dismantling the Imperial 70 (at least as far as I had to go) was straight forward, and I have a new respect for the construction of the 70. It's a very sturdy machine, and the craftsmanship quite impressive.
Whilst nutting out how the ribbon advance functioned, I discovered some gummed up parts which should have moved freely, and lots of cleaning with methylated spirit and synthetic light lube ( Inox) saw those issues solved.
Upon reassembly, the ribbon advanced perfectly. All up, including hand winding a new ribbon, the task was completed in 3 hours.
My sincere thanks to thetypewriter man.