After all the calculations it occurred to me that I checked the clock just when a GRS transit was occurring. After the clouds presented a hole to image through I got this: quite nice considering the horrible seeing.
July 17th 2007, 21h28 UTC. The transit was predicted 21h01 so GRS must have moved 16 degrees past the central meridian assuming it is at 119 degrees longitude. The tangent of 16 degrees=16/60=0.25 so the GRS must have moved about 0.25 times the radius of Jupiter, which seems to agree more or less with the image.
In addition a Europa and its shadow are visible!
A simulation/prediction of the GRS position at 21h28:
