My 18 month old combi boiler is leaking?

Hi, I have a Ravenheat Condensing Fanned Combi CSI 85T only 18 months old and it has started to leak. The boiler is situated in the airing cupboard in the bathroom. The plumber has been to this problem twice and each time tightened a nut but it still leaks, its coming from the hot water pipe. I have rung him again he says it may need a new pipe - surely not its only 18 months old. I have two basins to catch the water but after six days there is only a small amount of water in the basin, a small tear drop comes from the bottom (middle part of the boiler) but sometimes it leaks on the floor from the hot water pipe, this is a slightly thicker pipe with a plastic cover. Yesterday when I turned the cental heating on and ran the hot water it dripped over the floor. - can anyone please help. The boiler was installed on a grant through the Council and the guys that did it didn't have a clue they cut through a pipe when doing it and the water came through the ceiling. I have had other problems with it and because this was done for my Dad and he died in April of this year The Company who installed it Premier Gas, Wigan, said they couldn't do anything about it because it wasn't installed for me, so each time I have had to pay for the Plumber.

Thanks.

Josie