Several months later and only two weeks into our first trip in this caravan, near disaster struck.
  
The photo on the left shows a total separation of the left hand side swing arm from the axle clamp due to metal fatigue adjacent to the weld.   Since the fibre glass spring is a slipper arrangement only, this left the entire axle and wheel assembly attached to the caravan by the right hand side swing arm only.
This photo was taken prior to removal from the caravan and shows the axle is (fortunately) wedged forward of its original position between the chassis mounted spring and what is left of the swing arm.

It was only by incredible good luck that the left hand side of the axle / wheel assembly didn't drag back and under the caravan, lifting the caravan and steering it to the left, which would have resulted in an almost certain roll over.

This failure occurred only 3 months and 2,400Km after fitting.  The load rating is comfortably within the manufacturers specifications and the installation was inspected and certified by an approved automotive engineer prior to  re-registration as previously mentioned.

Hairline fractures in the opposite side point to imminent failure of that side as well.

Several phone calls from Victoria where we were stranded to Mr Stewart of Stewarts Super-Flex Suspension Systems in Qld and he agreed to send me what turned out to be a one off redesigned replacement swing arms.  The modifications incorporated significant changes and substantial  strengthening that I had demanded during the phone calls.   (During these phone conversations I found that this problem had occurred before but he claimed to have fixed it by merely relocating the galvanising vent holes.)

These parts were couriered to us several days later.   Repeated attempts to contact Mr Stewart over a period of several days concerning some fitting problems were unsuccessful.    
Even a friend back home who tried to call on Mr Stewart for me found his business premises closed and other businesses adjoining Stewarts reported he had been closed for nearly a week. 

The new replacement parts were subsequently fitted by a local firm and 28,000 Km later, we have had no further trouble.

On returning home two months later, I again repeatedly tried to contact Mr Stewart, but all I could find out from other businesses in the vicinity was that he had closed up the same week that I had my problem.

I have to this day never been able to contact Mr Stewart.       Where are you Mr Stewart ? you have a responsibility to to me and to all who purchased your product and who may be trying to make a claim on your 2 year warranty. 

Discussions following subsequent inspections by professional repairers and fabricators in this field lead me to believe, in the absence of input from the manufacturer that,
 this failure was inevitable and due to fundamental design flaws. 

Problem is, Mr Stewart sold many of these suspension systems and to my knowledge my replacement one was the only one to have been supplied without this potentially fatal flaw.
 
 

The problem with the original design is that the swing arms clamp tightly around the axle.   If the both swing arms rose and fell in unison, there would be no problem but of course this does not happen in practice. 

When only one swing arm rises or falls in relation to the other, there is a simultaneous pivoting and twisting force applied to the axle.    Since the axle is a solid 50mm square section and the swing arm is only a 50mm x 2.5mm square tube section, it forces the swing arm to constantly flex causing rapid failure.

The fix for this problem was to redesign the clamp end so that there is just sufficient clearance to allow the axle to twist and pivot freely thereby not placing any strain on the swing arm.
Relative movement between clamp and axle is small but enough to prevent the problem.
In addition to this, the swing arm was constructed from 4mm wall tubing with extra gussets connecting the swing arm to the clamp.

As a precaution the swing arm tube is sealed and to check the integrity of the assembly, the swing arms can be pressurised to test for fractures via a permanently fitted Schrader valve and pressure switch. 

Five years and 38,000 Km later, I have not had any further problems and no wear of moving parts (clamp & axle) is evident.

The fibreglass spring itself has never caused any problems and provides excellent springing characteristics similar to a coil spring but without the long term sagging problems.   It has the additional benefit of being non-linear in compression, initially soft but stiffening up considerably as compression increases.