I have the femme tens from babycaretens. It was fantastic in my first labour. Used it from first contractions until 8/9cm dilated then moved to G&A. Had no other pain relief. I plan to use it again this time and have just ordered new pads for it!
I used one from boots , I think it was only around £40. I used it through the last 4 months of pregnancy due to back pain and it was great for labour. I have used several ones from the hospital/ physio dept over the years and the boots one was just as good.
I'd also recommend the boots one, I've just ordered new pads so I can use it with #2. Here's a link for it: http://www.bootsmaternityrentalproducts.co.uk/product.php?pid=Boots-Maternity-TENS-Kit--30 You can also rent them for £31