What are my rights if I am unhappy with the work carried out on my boiler?

I recently had a boiler fitted to my house, however the plumber put the outlet pipe (which blows out the hot air) to opposite to my soil pipe (which would melt with the hot air). He pointed this out, and advised that we could buy an elbow extension which would blow the air in a direction away from the soil pipe. We have bought the extension but it will not fit as there is not enough space between the outlet pipe and soil pipe. Is he liable to resolve this, or do we have to pay him to resolve it? I live in the UK.