In case anyone has the same problem...
The twin blades are supposed to be at 90 degrees to one another so they do not touch when they rotate.
But I hit large protruding bank of soil / stone with one of the twin blades which caused it to jump out of alignment, and collide with the other blade (one hell of a noise).
Here is how to fix it (provided you blades are still in one piece):-
The two blades are kept in sync by a toothed belt which is covered with a plastic cover.
Undo the plastic cover.
Loosen all the bolts / sprockets that hold the toothed belt tight so there is enough give in it to allow the blade to be twisted back to the right position without the belt moving or anything getting sheared off.
Tighten everything up again.
If you have the right tools it should only take about 45 minutes..
NB - while I am writing this, yes it has to be said that the build quality / design of this mower does leave something to be desired.....