This worked well for me, Mark Powell, think it was told me.
I lifted the carpet as it was near a corner and cut out the
underlay.
50/50 Fibre and Fabric rinse and hot water and dribble it
over the stained area.
Dwell for 20 minutes then extract that was on wool as well.
You could, if you looked really hard, still see a very slight
discolouration but at a glance nothing.
Oh and don't forget to cut them another piece of underlay.
John