Poor Quality Installation

HMD Gutters installed gutters and down pipes on my premises. The gutters and down pipes come with a 15 years guarantee and the contract is binding as stated on his invoice. <br> <br> The products was installed on the 23/07/2015. With the first rain in december 2015 all of the gutters started leaking and the pipes are not draining at all. We have contacted the persons responsible and they are refusing to even come and take a look at their faulty product. We have paid the amount in full. <br> <br> They are aggressive when confronted and refuse to repair their faulty work.