Need help asap with seriously Leaking Baxi Duo Tec Combi boiler.?

Is it possible that you actually have water coming in from outside from the rain, possibly a change direction in the wind blowing it into the flue. I have seen this happen before so may be worth a look. Apart from that stick to the manufacturers repair offer as they know the boiler inside out and will give you 12 months warrantee which I dont think BG do. If your boiler was leaking it would be all the time if it were the hot water side and if it were the heating side it would leak until the pressure went flat and then maybe a drip for a while but I cant see it starting and stopping.