AFAIK Homebond only covers the FIRST owner for the first 10 years. As you have stated the house is second hand, despite the fact that it is only 7yrs old - you are not covered. [broken link removed]
What did your plumber use to clear the drain - rods? Did he survey the line with a camera unit? If not, I would recommend hiring a drain cleaning company and get a camera survey done. Should cost ~€300 but you will get a detailed report and tape showing exactly what the problem is.
If there is a major problem and you need the drains replaced or relined, then your home insurance policy may cover the cost of same (and indeed the camera survey). You will need to check with your insurer.