SO, I have gone and bought a TENS machine off ebay. It cost me $25 (much much cheaper then hiring one). It is a regular TENS, EMS machine, rather then a labour TENS macchine. But it has 9 programs in it, plus room for adding three programs myself (if I can figure out how), and although it doesn't have a 'booster' button like the labour specific machines, it does have an intensity button on the unit which can go immediately from lower to higher intensity and back again (so it sounds like the same thing). It has four large electrode pad on it too (with 8 more spare). FX that it is as simple to use as it appears. I will take it into my physio and get them to go through it with me too after I get it and have a play with it myself.

If worst comes to worst, and for some reason it isn't appropriate for me to use in labour (or I don't get the chance/ or don't like it) then it was still nice and cheap (have wasted more money buying lunch at the food court) and DH can use it for his recurrent back pain.