Funnily enough I just bought something for that very purpose, and it's turned out to be quite handy.
I tried out Chemical Guys - Strong Wash (Bug Bugger):
http://www.cleanyourcar.co.uk/washing-a ... d_183.html
Think I may have diluted it a little too much but even so leaving it on for a good 10 minutes helped me shift some tar that has been a pain for quite some time - didn't shift it all mind, I'm hoping a few more visits and a slightly higher concentration should do the trick - for the flies, it was spot on.
There's probably loads of similar things about but that's one I found useful anyway.