Subsequent bends

After a little bit of research, it was apparent that packing sand tightly into a tube would support the tube walls during bending.  Some people use heat in the process, and some people warn against using heat.
We decided to do ours cold.
We welded an architectural steel ball into the end of the tube, inverted the tube and filled it with silver sand.  The trick in doing this is to pack the sand down as tightly as possible (we rammed, hammered and tamped with vigour).  Once completely full, we hammered another architectural ball into the open end and welded it in place.
Now our tube was ready to bend.

             

The process worked like a dream!  With a hydraulic pipe bender like this, you dont need heat.  And although I am sure that flushing the sand through with water would certainly pack the sand tighter, we found that we could hammer the sand down perfectly tight enough to give us minimal flattening during bending.