Serious Drains issue after Extension Built

I had a rear single storey extension built a few months back, since then my drains are constantly being blocked. I've opened the shore to the rear of the house to find it full of small bricks and a gravel like substance, I clean it out but it's full a week or so later, a builder friend of mine said it looks like hardcore.

The shore in question is less than a foot from where the extension was build, the drains would run from the back of the existing house parrallel with the extension all the way.

Any ideas as to what is causing this? I suspect a pipe was either knocked out of place or has been damaged while the foundations were being dug, however I have no idea how to prove this as all the pipes are now under concrete.

The builder turned out to be a right cowboy as you may have noted from all my other posts over the last few months, he's denying this could possibly be anything to with him or the foundations and has blamed anything but himself.

Any advice on where to go from here would be greatly appreciated.