Hi everyone.
*** SUCCESS ! ***After a lot of trial and error I have managed to find a way of getting a near perfect
90 degree bend in both ERW and CDS using a cheap clark type bender. There is still one more experiment before I feel I can say with confidence “this works” – but I am very near (see below).
Thank you to everyone who made comments / posted links etc - it has all been invaluable to my understanding. I am sure there is still much more to learn 
A few extra components are used to get these results with the bender, these are:
• An adjustable clamp
• A pressure shim
• A bending shim
• 2 roller followers
Here is the technique I have been using:
Create a pressure clamp.a. Drill a hole into the former centrally below the forming curve
b. Create a strap to fit over the pipe that can be secured with a bolt through the hole at (a) – This needs to be a high tensile bolt or it will bend. I used the inner of a rawlbolt.
c. Weld a nut to the top of the strap and fit a bolt so that the pipe can be held firmly in the former.
Create a pressure shim• This shim will go between the pressure bolt and the piece to be bent. This stops the bolt point pressure dinging the piece in.
• Cut a short length of pipe / tube with about a 45 degree arc, and spread it so it follows the Outside of the work piece
Create a bending shim • Take a length of tube / pipe sufficient to line the bending former.
• Cut a 110 degree section out of the tube / pipe. I did this then chopped the length off as its easier to handle in that sequence
• Put the open slot of the piece over a piece of same diameter tube and wallop it with a hammer to force it open. Eventually with care the cut tube will nestle nicely on the outside of the piece to be bent
Create roller followers• Both rollers have a follower between the roller and the work piece
• This is formed in a similar manner to above by cutting down the length of a tube to create 2 X 180 degree pieces
• These are opened up to fit by the delicate use of a big hammer as above.
Fill the pipe with sand. This is achieved by• Sealing one end with insulation tape, - yes the pressure at the end will be that low!
• Filling the pipe with water
• Carefully putting wet sand into the open end until the pipe is full of sand and the water displaced
• Tamp down the sand inside the pipe / tube with a wooden dowel
• Put in more sand
• Seal the open end with tape
Assembling it allPretty much as it seems, I oiled everything that moved as they went together
TechniqueThe bending shim starts off straight below the work peice, and ends up bent into the bending former. As the bend is started the work piece will move down into the bending shim. I regularly took up the slack with the pressure shim and bolt. Eventually the forces reverse, and the centre of the work piece will try to rise out of the former, thats when the pressure shim does it's work. I started on the outermost roller, then move in one set, then moved in again, and I took my time
ResultsThe pictures below shows the components assembled with a length of CDS in the bender bent to 90 degrees.
ConcernsIn each instance I started with a straight bending shim and allowed it to bend down with the pipe / tube. I have not yet tried a second tube bend using the preformed bending shim.
