Load of tosh - LR fitted steel dowels replacing plastic when they had early TD5 engines in for blown head gaskets. LR dont even sell the plastic ones any more.
Steel everytime.......
If you are really tight and have access to a lathe you can make your own. Important they are a nice push fit in the head without being too loose or too tight. Length is not so critical other than dont make them so long that the head wont seat. Having said that they are not that expensive to buy.