Honestly, your best bets at removing tar is by using either Autoglym ITR or getting a hold of Autosmart Tardis.
I personally use Autosmart Tardis, it's really strong and once applied you can literally watch the tar dripping off the paint...trust me its good. You can get a hold of the stuff through
Polished Bliss - Car Care Products - Car Care Advice - Detailing Services within their 'clean' section of the 'car care products' section
Hopes this helps, just my 2 pence
