Well I need to give a big shout out to @Phillo who recommended using paraffin (kerosene for you American folk) to get the tar off. I'll be honest after trying to get it off with regular washing the moment I got home after being sprayed with the tar and it doing absolutely NOTHING even when scratching with a nail I wasn't very hopeful. Then it took me weeks to finally get around to it this weekend and I think now it definitely won't work because I've left it too long. Used an old cloth nappy and liberally doused it in the stuff as if I was making a petrol bomb and then laid it across the windscreen for a few minutes and would your believe everything slid right off with virtually no effort or elbow grease required on my part. Getting the cloth to stay in place around the little fairing bits and the heads was more of a trick but even those cleaned easily and then I put it through a regular wash (in the backyard out of the public eye...water restrictions be damn) and I swear the bike is cleaner now than when I got it. Of course I washed the whole bike only to realize after the fact that I missed the tar on the mirrors, but fortunately you don't see that too obviously so it will wait for another day. However the massive failure on my part is that I always put my wash mits and micro fiber towels through the washing machine and then seal them in a bag after I'm done...and in doing so I completely forget about the paraffin nappy. To make things worse I then filled the washing machine with clothes from the washing basket...and now my wife wants to murder me because everything smells like paraffin. Fortunately it seems easy enough to reverse with some sunlight liquid and a second wash in vinegar and bicarbonate soda.