thanks ladies-
one more question- can i use the tens at the same time as the gas, or will it get too confusing! iykwim? with all these things to press/suck etc.
You should be fine with both, but I used a TENS which was automatic so didn't have to do anything with it during contractions. It was on all the time at the same rate and I could turn a dial to turn it up rather than press a button at each contraction (It was a sports med one rather than a maternity one - same basic unit.)

I ended up using the gas and my TENS remained on during that time. It could be difficult to managed both the gas and the TENS but honestly, by that stage, you will do what you need to do and I imagine, wont be too worried about both.. exactly as Falguni said...!