I'd like to know more about the rug. Origin, construction, fibre, colour, age, condition. Also, who says it’s worth £15000. You need too be very clear in your mind what you have to deal with here. Some people have fanciful ideas about the value of their rugs. E-Mail me a photo and I'll try to ID it and give you a guide.
I'm not saying the rug is not worth that amount.
It could be a modern hand made rug, say 20 to 30 years old, and very fine. Assuming you will move it, be very careful. If you need to fold the rug do so with pile outside to prevent the weft threads from breaking. If you roll it, put a pole up the middle to stop it bending when you lift it. Before you take the rug away point out any damage that’s apparent, particularly the fringe. Certain types of fine Persian rug have bleached white fringe (warps) that will fall to pieces if you try to scrub it.
As for the cleaning. I would not recommend cleaning on site. It's likely to be quite mucky and need beating back and front to remove the grit and gravel.
Best of luck