This is a common problem due to poor installation methods. The Flue should have been installed with a slight angle going down to the outside so any ingress of water from adverse weather conditions including dripping gutters will just simply run back out and cause no problems. I would have a guess that you're flue has been installed in the horizontal position or even slightly uphill running back into the boiler. You need to either have the flue re-installed correctly to permanently cure the problem, or have a word with you're neighbour and tell them the damage the faulty gutter is causing and ask them to fix it. This will not however gaurantee it will happen again with heavy rain and a prevaling wind. You also can not cover it up as it is designed for the safe evacuation of the products of combustion and the appliance will be condemned.