My Leon friend rang me last night he bought a set of glow plugs from a local auto electrical dealer. It took a while to get the old ones out a quarter of a turn in and out. He did a search on Seat forums and the number of users that have stripped the threads by constantly unscrewing these. When he got them all out he checked them with the new ones listed for his model and his old ones were around 8mm longer. He took them back to the shop in his partners car and they checked the numbers and they had Audi glow plugs in it so he had a set of those.
He rang to ask if I could reset the MIL light as its still on. I did a full scan and the same fault showed. Now a little tip with VCDS if you double click on the red writing in the results screen it will remove that DLC error only. After a few stop starts it appears to have cured it, only time will tell.