I redid it a number of times, and could not improve much. I turned to the internet and found this great link (http://sewing.patternreview.com/SewingDiscussions/topic/51298) where Belinda offers the following advice
The twisting you are encountering is most likely due to one of three things.
a) Binding strips cut off-grain. Pay very close attention to both grains when cutting your strips, and be very accurate. Many stretch and knit fabrics can be off-grain. Some are even knitted off-grain making them totally useless for binding. Off-grain strips will always twist and turn binding into a frustrating nightmare.
b) Poor quality fabric. Even if the fabric is on-grain it can still be of a poor quality and cause issues.
c) The wrong differential feed settings on your machine.
I was using good quality lycra. I checked that I was cutting on grain. I tried changing both the width of the binding strips and the amount of differential feed. Still no joy.
Up until this point I was using the binder that came with my machine, which makes 1/2" binding. I have also bought a number of industrial binders to make bindings in other widths. I was tearing my hair out at this point and decided to switch to one of the industrial binders....and woo hoo, look what a fabulous job it did.
Admittedly, the binding fabric was a different lycra (because I was rapidly running out of blue lycra on my samples). Time to try again on the real thing.
Here is the smaller shirt. Not perfect, but much improved.
Here is the larger shirt.
I thought it was interesting that the same binder, using the same width strips, made binding of a slightly different width in the 2 different lycra fabrics.
I'm guessing, but can't be sure, that the success was not due to using an industrial binder instead of the binder that came with my machine, but rather due to making a narrower binding. I have had trouble with another project that I am yet to show you, but I am thinking that width of binding is important for some fabrics.
For those interested, I bought my binders from ngosew on ebay. This is a different supplier to the one mentioned in many of the PR discussions. They are a bit cheaper, the supplier has answered my email queries, and I have had no problems with delivery. I attach the binders to my machine with blu-tack.