Chat to a SEAT dealership about this, in the past you were given a paperwork version of the service documentation, and on the stuck on "build data" sheet would be lots of things including latterly, the "g/km" - maybe that
sticker is also stuck to the side of the spare wheel well, worth checking, but if V5 says otherwise you will be fighting an very steep up hill battle as government departments will never admit that they were wrong, also that V5 should have been generated from info that the original selling SEAT dealership gave them, maybe a mistake was made there, but probably no way to resolve it now.
Just to note, I have a 2011MY Audi S4 built in late January 2011 and sold in mid February 2011, it get listed as being a 234g/km output car, lots of same 6MT S4s slightly older and slightly newer got listed as being slightly lower g/km out, so fit into about half the £555 and year road tax bracket I pay, life sucks at times - probably too late for government to move to a much lower rate of annual road tax and push the tax on the fuel used so that the actual polluters, ie total theoretical g/year is what we are hit for!