Aphids in Trees

Our oak tree overhangs our drive and every year a sticky substance is deposited on our cars, which I have to wash off twice a week. I believe it is secreted from aphids and that there is an insecticide which is taken up by the tree roots to alleviate the problem. I think it is called Bayer Advanced Garden Tree and Shrub Insecticide concentrate, but I can only find it on USA websites. Does anyone know if it it available in the UK or is there an altenative? The tree is protected so I can't take the obvious solution! Many thanks..