I have this on my 09 plate
K1. Exact same
problem.
A dull knock, only noticable at low speeds, less than 15 mph. only needs slight road defect to set it off (cats eyes, drain covers, thick white lines etc).
I had mine back at SEAT a couple of months ago for it, and a couple of other warranty issues. They said they took various pannels off and found a couple of bits of loose trim which they padded and secured.
I was that pleased with the other major work the dealer had done, I failed to notice the knocking was still there the next day. They were convinced it sounded like loose trim, but im in agreement that it sounds like a loose suspension component at the rear.
TBH, they make such a meal out of doing any warranty work, its put me off going back and im just going to live with it untill about a month before the warranty runs out, then I'll take it back and get that and any other work carried out free of charge before the first MOT is due next summer.
If you find out exactly what it is, please post back, with photos if poss.