Condensing boiler flue gas condensation

I have recently, about two months ago, had a gas condensing boiler installed. I now find that the outer surface of the flue where it passes through the upstairs of the house is showing signs of the effect of dampness. I understand that this is the result of the flue gas lower exit temperature and the consequent condensation of water vapour within the flue coming.through. I wish to avoid any expensive remedy such as lining or replacing the flue, and therefore am seeking an effective solution to this problem which does not entail expensive building works on the flue.

Could this be condensation forming on the air intake pipe due to cold outside conditions and moist air indoors? The flue pipe itself (possibly the inner of concentric pipes depending on the flue design) should not have any moisture on the outside of it.