Hi,
Having similar issues up here in Glasgow.
Zebra pad on a buffer with a powerbust/citra boost lifts it mostly.
Ive tried lots of solvents - olefins, chlorinated and alcohols but its not an easy one to lift in one hit. Trouble is that you take the dirt off the surface and it looks to be away, but isnt.
Does Doug Holloway still frequent, i would love to pick his brain on this.
Graeme