Ok, so the 'make your own garter bar' instructions from a load of hair pins and a metre rule turned out to be a lot more fiddly than it sounded! Apart from which, the hair grips pictured were far too big for the needles on my machine. Reading up on it a bit more it turns out that manufacturers didn't used to produce garter bars for chunky machines. I'm assuming that this would be the only sensible conclusion why someone would want to try to manufacture their own in this way.
After this I phoned round the existing knitting machine manufacturers, knitting machine spares shops & looked at the Guild of Machine Knitters website to see if I could find any for sale. The resounding answer was no, they stopped making them a long time ago and they are very rare and I would be lucky to find one.
This only made me want one more....I am desperate to experiment with this technique. I resorted to e-bay, where there were a couple (mostly by international sellers). I decided to splash out on a full boxed set with instructions. It should be arriving in the next few days! Now I only have to learn how to use it.