I'm going to change the P-gasket behind the water pump next week and thought I may as well do the core plug next to it at the same time, but for the life of me cannot find any reference to it's size.

So does anyone know what size said plug is? The engine is a 300Tdi.

erc4996 is the plug needed ,loctite outer edge and ensure it sits just in from flush using a loosish socket

Spot on cheers.

Any particular reason you recommend loctite? Reading various threads and articles I've seen everything recommended from nothing through blue hylomar to red setting hylomar but you're the first person to recommend loctite.

EDIT: I'm assuming you mean retaining compound like loctite 638 rather than thread locker.