I rented the Rug Doctor on Thursda, only just remembered to come and give an update though.
The Persil Small and Mighty did a great job, on both the carpet and on my sofa with the upholstery attachment. Everything looks really good and it smells lovely now too.
On the Rig Doctor bottle of detergent it said 150ml per 9litre bucket of water, so I used about half of that in case it made too many bubbles but I needn't have worried. It worked best when the water was really hot, as hot as it can come from your taps, once the water had cooled towards the end of a load it wasn't as effective so the trick is to do the dirtiest bits straight away with fresh water.
For the sofa I found that if you press the button to spray the water on the upholstery 'wand' and put a fine mist over the section you are working on and then go over it with the wand and suck up the water it works best, otherwise you end up with a very soggy sofa.
It has given our very heavily soiled living room carpet almost as good a clean as when we paid a professional to come and do it, not quite as good but close. It was well worth doing because for less than half the price I have done all the rooms in the house plus the sofa. The disadvantage is that it doesn't do very well right up against the wall/skirting board. Where I could get away with it I left it, where I couldn't I had to use the hose wand attachment to get right up to the wall (ie. the last 2cm to the wall) but it doesn't do as good a job as the main cleaner.
You do need a separate bucket to fill up the water reservoir, I thought I could save time and just fill up the water collecting bucket and use that to pout into the reservoir - you can't, it doesn't pour well and I ended up with about 3 litres of water all over my living room.
The machine is smaller than I thought it would be and easily fit in my car. It was lighter than I expected too, though don't try to carry it upstairs when full of water, its heavier than you would think!
The other thing is that unless you have a big house I would imagine a one day rent would be enough. I did the living room, stairs, landing and three bedrooms in about 4.5hrs, no breaks though.
I'll definitely rent it again and do the same again. One day rental was £19.99 I think so its very cheap when you think the all the carpets in the house have been done.
Hope that helps somebody!