have you tried those acid washes they use for cleaning brickwork? I've never used one but they are supposed to be good, robbo will know better.

i am at a bit of a loss to another answer apart from what's already been said. only advice that i can give is that if it is polybond then it will be clear when dry so i doubt that it is polybond.
i personally think it may be a resedue that has bled from the cement, therefore maybe an acid brick cleaner.
then move on to more brutal methods...jetwasher first, no joy then the sand blaster, stil no joy, then a needle gun. the needle gun is really a last resort because it is an animal and it will actually take some of the face off the brick it is also unsuitable for a lot of brickwork

i must stress that sand blasting and the needle gun are very harsh methods, so only use on a last resort