After doing a bit of reasearch it does turn out to be a reamer for cleaning/recutting the Glow Plug apeture and seat. 01731000 - Glow Plug Reamer

I assume that the tool (the reamer?) screws into where the glow plug would be. Sykes Pickavent give three size options. 01730800 - M8 x 1.0 Thread 01730900 - M9 x 1.0 Thread 01731000 - M10 x 1.0 / M10 x 1.25 Thread.

I have checked the forum but cannoy find the answer. After some start problems I changed the glow plugs and now is a lot better. I have now found out that I could have cleaned out the glow plug ports in the engine block. I have seen some cleaning tools advertised but they come in different...