They fixed it and appears to be working fine now. Maybe it's me but it seems like they flashed the ECU or something because I noticed that the instant consumption meter, wich is the option I usually have selected in the dash computer, is now being updated very quickly. I also did notice that the mileage improved very sightly.
They had to disassemble the head and the turbo intake to get to the injectors; they replaced all gaskets there and those of the injectors. Plus all the mounting bolts of the common rail. Like the rest of you, the problem was that one of those screws fatigued and broke at the head. They didn't tell me which one of the bolts it was, but I have no reason to think it failed differently than what we already now (the one near the first injector).
They charged me GBP 60, which I consider reasonable for a repair of this kind. Volkswagen of Mexico (they distribute SEAT over here) and the dealer paid for the rest. I service it in the same place I bought it and intend to remain doing so; perhaps they wised up and are trying to keep me happy.
Thank you all, you've been very helpful. This is certainly a widespread issue so I'm still calling the consumer authorities to report it. I'm satisfied with the solution given, but I don't want to hear from someone else to go through this. Fuel leaks are very dangerous and should be approached proactively.