I can imagine that is a really nerve-wracking situation to be in.
Have you taken a look at our Driving and Parkinson's booklet? It was updated late last year and you can find it here: http://bit.ly/PiairU
If you have not already, do have a read to see if it might help. Also please keep us updated as there are a lot of people here who are very concerned about this issue.
Best of luck and I hope it helps.