On a purely historical note:
03: is the danger area descriptor - in case there are other danger areas at this latitude (this is an assumption on my part...)
In the dim and distant past when the current system of numbering danger areas was first adopted the second and third digits represented the numbering of the areas within the latitude band, from west to east.
Thus D001 was off St Mawgan, and still is, D002 was in St Austell Bay, but is no longer in existence and D003 is in the Plymouth Exercise areas etc. However with the deletion of some areas and the addition of new ones the system is no longer coherent.
YS