Hi there,
I've just hired a tens machine for my upcoming birthing experience and was cheap...and come recommended. For 7 weeks it cost me £22 and they send out spare batteries and pads and a freepost envelope for return. Also if you go over your time you don't get charged extra. Boots tens hire was £35 for 4 weeks so this is much better!
www.tens.co.uk/
Central House, 37 Gate Lane, Boldmere, Sutton Coldfield B73 5TS UK
Tel: 0121 355 6555 or 0845 0900 800, Fax: 0121 355 8787
I called them and spoke to Craig, but I think Martin also does the trick too.