Ian, I'm a self-employed sales agent for the RAC. They book all the sites, not sure how much it costs per day, but they pay tesco's, sainsbury's in the region of a few million each for the privalge nationally. I only earn commision which goes up if you hit targets, but you also get hit with clawbacks if anyone cancels so it can hit you serveral months down the line. Not bad earnings over the summer months but the winter sucks, standing outside in the freezing for upto 8 hours. I am currently averaging about 2.5 sales a day.
Not sure if they would allow you to promote carpet cleaning or not, quite a few of the suppermarkets hire out carpet cleaning machines so might be seen as competing against them. You could always offer a free carpet clean to the manager in return for having a stand for the day. I expect it would work better if you could do a demonstration or have samples of before and after squares of carpet.