I'm just back from re-treating this carpet using Ultrapac @ 50:1, long dwell time, 230psi (hot) extraction. This improved the staining, but did not completely clear it - it's about 50% improved. I even went over the really bad bits with my stair-tool - which pulled out a LOT of carpet!
Anyway, a great learning experience
AND... This particular customer owns an aluminium shipping company, he saw me struggling a little loading on the first visit, so when he knew I was going to re-do the cleaning for free, he made me this fantastic, custom made alu ramp - RESULT!!!!