Not 100% sure, but I think the ceramics were long gone by the time the 4.2TDI came. They were only used for a year and were a disaster as they would shatter and ruin the engine. The fix was to fit steel glow plugs and recode for a lower voltage 4.4v vs. 12v
The ceramic ones had a white band.
http://www.vwaudiforum.co.uk/forum/a.../t-134034.html