Howdy Ian, I am an old fart with a bad back and would love to try a scythe but I am not sure if that side to side movement would be good for me.
I was just wondering if you had any tips on stance, movement, warm up exercises, etc.
And whether there might be any sort of ergonomic scythe that would be easier to use ?

Howdy do, Miles. Since I don't know the specifics of what's up with your back and I'm also nowhere close to being a doctor, please do not take the following as medical advice and proceed at your own risk. The metaphor I often use (and do so in the book) about the ideal mowing motion is that of a Japanese pellet drum called a den-den daiko. It is a small drum with a pellet at the end of a string hanging from each side of the shell. A handle sticks down from the bottom of it. It is played by using the hands to rotate the drum at the axis (the handle). The pellets then swing about, striking the heads of the drum. When you use your body this way (swinging at your axis; your spine), your arms swing through the air effortlessly. A bit of extra tension in the arms to stabilize the scythe is all that is required of them.

This is all explained in more detail and with illustrations in my book. Depending on your back situation, this may be a painless motion for you, or it may not. Proceed with caution and easy does it!

I too have a bad back.  That gas guzzling ride on lawn mower is hell on my back.  The scything motion looks very relaxing and I think if done gently, it could actually benefit my back problems as it's very close to the motions the doctors recommend.

Ian's book has a whole chapter on how to care for our body while scything.  He describes scything as a dance that can be gentle and joyful for the body. 

One of the tools for keeping our backs healthy, Ian talks about in his book is the Alexander Technique as a way of retraining our body. 
I really like the idea that scything is an opportunity to teach our bodies how to move in a gentle way.

As a massage therapist that does a lot of scything I answered a comparable question from someone with scoliosis.  The motion of scything is the same of walking but with the emphasis on the swing of the arms and limiting the motion of the feet. The primary muscles used scything are the Latissimus Dorsi which run from under the upper arm to the lumbar spine. There under development or uneven development often contributes to a bad back.
If you swing the scythe by pulling with the left hand and swing it back py pulling with the right hand and step forward about 2 inches with the right foot before starting the swing and with the left foot at the end of the swing the normal walking rhythm is maintained which can be very rehabilitating for the spine. That is what I do for myself.