They draw next to no power so no problems there, provideing that you don't wrap them up in tea towels lace doillys or the like they don't over heat.
Thing to bear in mind though is cos they're relatively cheap so is the power supply. Switching that on and off is going to reduce its life, and while you may think that's not really your problem as you rent it all from ntl, it would be a bummer if it died on a Friday morning and ntl couldn't get out until the next Tuesday or Wednesday
