Leak in kitchen...who's liable?

[WhatAFunnyPotato]

We bought our first house in March last year. A survey found a damp issue in the exterior wall in the kitchen, flagged up as a problem with the exterior render (rain water leaking through cracks). We agreed the sale on the basis that the seller would arrange to have the render repaired, and money was withheld until such time as it had been completed.

We had to agree that the work had been carried out satisfactorily before the money was released. It seemed so, although we hadn't had significant rainfall to test it.

Fast forward to this winter. We now have the same leak in the same wall. What should I do? I assume because I agreed the work had been done that I can't go back to the seller? What about the workmen?

I don't want to pay for expensive render repair again...

[WhatAFunnyPotato]

Anyone...?