Sorry i have just realised that I should have posted this in the Mendel Forum.
Having read a similar post there, I'll check if the e_steps_per_mm are set high enough then post a video tomorrow if the problem persists.
Yes, as a test, when we type "extrude" in the print window, the hot-end deposits a thin filament but it accumulates in the same place because the carriage does not move (should it?). The hot-end temp is set at 205 C.
When we print a simple STL file, the carriage moves layer by layer but the extruder deposits nothing and the Extruder-motor is motionless.