Hi i just recently had the same
problem with my seat leon fr and it wasnt the glow plug that caused it, it was the butterfly hinges or clips in the intake manifold which have came loose over time and gone in to the engine (firing chamber) and then broke the glow pug which damaged the piston head the chamber 3valves and the head! I had to replace the engine which wasnt cheap but i took the butterfly clips out if the intake manifold now as ther pretty useless anyway.