Leaking porch roof

Had two tradesmen from checkatrade, built a new porch for us, they said roof was solid and secure. They also stated the roof didn't need replacing. When they first came to do the job, they put the steel posts up to hold the roof in place. When putting the steel posts up, one of the posts went right through the roof but apparently didn't break the felt. After this they put up a false ceiling in the porch to cover the damage. When we asked afterwards what the dip was in the roof the said we missed the joist with the steel rod, but did confirm they would put a false ceiling in to cover it and the felt had been not broken. They put up the false roof and it was ok for approx 2 weeks until it rained heavily. When it rained heavy the roof was leaking really bad. We rang them just before Christmas to complain and they never came back and we never heard from them. After Christmas, I rang again and they said they would come in the next couple of days, they did with another guy who I had not seen before, he said your roof is old and rotted tried to say it was our fault. It had never leaked before the work was done, and the workmen at the time said it was a solid roof and did-not need replacing. I will not be letting this drop and would like your professional opinion on this matter. Many thanks