Refusing to refund a faulty product!

We recently purchased a gas hob that was on a Black Friday special. When we got it home and had it ready to install we found that the igniters were not all working . Disappointed that we couldnt complete our Reno - we took it back to builders and they exchanged the product for us. This second hob had the exact same issue - we checked before getting someone in to install - but again the same issue! We go back to builders - they even ask us how many we have brought back - this is obviously a whole batch that is faulty! They refuse to give us a refund saying a technician will come out on Saturday or Monday. No one arrives on either day and no one even calls. They then email saying we cant get a refund as we didnt get a technician to install and dont have an install certificate - We never even have gotten to the point of installing because the igniters were faulty. Honestly i am disgusted in this after service - to sell faulty products and then make it impossible for us to return them is just plain wrong