My initial post from Aug 17 has been locked. (over 3 months)
A surveillace camera found 2 major blockages.
A 90% blockage near the back-door of the premises. It's a 4" pipe!
That requires excavation.It looks like a caved-in duct, with plastic material surrounding the duct causing part the fault.
Another 50% blockage is just outside the front door(under 3 terracotta tiled steps),the main entrance to the building.
This might be cleared by water-jetting.It looks like grease build-up and paper towelling.The engineer also told me they would try to make a repair to this area without digging up the tiled steps, as this would be a 'nightmare'.
I viewed the video this morning, on the engineer's little portable monitor.
I will be informed by the local housing association when work will start and of the costs involved.