Well, the stuff definitely works.
Used it in another conservatory roof yesterday, applying it neat, not leaving it to dwell at all. Just started scrubbing all the upvc and it can up like new.
The area I was most impressed with, is the plate that the tallest finials sit on. These always have a horizontal line of grey going around the edge of them. It removed this in seconds.
Verdict is in TFR works but is bloody time consuming to use as the suds take forever to wash away.