Hi Nikki,
I used the Elle Tens and it was amazing. I had a painful back-to-back labour and managed on TENS and hypnobirthing alone to fully dilated and for several hours in second stage. It's great because it has 15 levels at Boost 1, then another 15 at Boost 2, with upper and lower electrodes adjustable separately. Then finally you can go to Optimax. The huge range of levels give you a real feeling of control, e.g. when on Boost 1, level 4, it really helped me that there was so much more still available.
I did get the large electrodes and found them to be excellent. You use the normal sized ones for the top, and replace the bottom two with a long strip which goes right across the whole width of your back.
I bought from tens.co.uk. Excellent, next day delivery, and I liked the free practice electrodes and the option of the long strip, which you don't get elsewhere. I decided to buy as we can use it again if we have another baby, and my H can try it for sports injuries too.
I don't think that Tens would be nearly so good for a labour which isn't back-to-back, but it's worth getting whatever the position of your baby, as my son only turned back-to-back on the day I went into labour.