I'd borrow some more, £400 will get you a hire machine what anyone can get from a supermarket or dry cleaners.
You will probably get a 35 psi pump and one small vac motor a very wet and partially cleaned carpet, in the walkways you won't make too much of an impression as it won't cut through the dirt.
No laughing just an honest answer, if you want my opinion put the job off for another week drum up a bit more business and then spend, you 'll only spend once and you'll leave the cleanest of carpets behind with your money and with no problems.
Shaun