8R0877459A is indeed the correct part for the Seat leon ST. (It also fits the Audi Q5)
Just as an update to this thread, my sunroof started leaking again in the same place as last year.
After investigation i found that the outer sunroof seal was again coming away from the bodywork, allowing water down the back, again
As it was still under warranty with the repair done by the sunroof company they provided me with a new seal, and as they couldnt fit me in until after the new year i decided to fit it myself !!
It wasnt that hard to replace to be honest and i was able to do a better job that the sunroof company who replaced it last year for me as i was able to take my time and do it properly.
I cleaned off all the old residue of the old seal by hand and the offical VAG cleane, primed the area where the new seal will stick, then installed in using a hear dryer to heat the bodywork and seal up whilst i stuck the new seal down.
Complete success.
Part numbers.
Seal = 8R0877459A
Cleaner = D00940104 (in reality just rubbing alcohol)
Primer = D00920002
