James,
Almost on a daily basis I get tricky carpets and fabrics to clean and I face the decision you have with this carpet. It is very difficult to offer definitive advice when you are not on site and you can't touch the carpet or properly inspect it. I am sure that no-one here would wish to advise you to wet clean it and then for it to shrink. Like Shaun I am pretty convinced I have come across this carpet on stairs before and like most people I was pretty sure it would shrink if it was overwet or not installed correctly. All of the signals are that you should walk away or dry clean the carpet. You should consider strongly this option because it is only 1 staircase and you will regret it if anything goes wrong. If I wanted to wet clean it I would take the following steps,
1. Check the installation is good. If not make it good or do not proceed with a wet clean
2. Dry vac
3. Mist 1 of the steps with microsplitters and towel and leave for 10 mins to check for shrinkage
4. Extract this 1 step using a low moisture hand tool (cfr tool) and leave for 10 minutes to check for shrinkage.
5. If all goes well with steps 3 and 4 fire away cautiously.
6.Use an airmover to dry
7. Breath a huge sigh of relief
Good luck and let us know what you decide to do.
Nigel
www.designcare.co.uk