Roy's Roads. Linked information
O.S. All-numeric grid referencing
Numeric grid referencing omits the reference letter prefixes, eg NC, NG or NS, and replaces them with a number prefix: one extra number added to the Easting and one to the Northing.
The zero reference for the numbers is somewhere SW of the Scilly Isles and the increment is per 100Km corresponding to a new letter-prefix.
NC = 2_ _ _ _ _ 9_ _ _ _ _
ND = 3_ _ _ _ _ 9_ _ _ _ _
NG = 1_ _ _ _ _ 8_ _ _ _ _
NH = 2_ _ _ _ _ 8_ _ _ _ _
NJ = 3_ _ _ _ _ 8_ _ _ _ _
NK = 4_ _ _ _ _ 8_ _ _ _ _
NM = 1_ _ _ _ _ 7_ _ _ _ _
NN = 2_ _ _ _ _ 7_ _ _ _ _
NO = 3_ _ _ _ _ 7_ _ _ _ _
NR = 1_ _ _ _ _ 6_ _ _ _ _
NS = 2_ _ _ _ _ 6_ _ _ _ _
NT = 3_ _ _ _ _ 6_ _ _ _ _
As an example, a conventional ten figure reference for a spot in Perth city centre is expressed as NO12100 23900. This converts to a new 12 figure reference of 312100 723900 , with a one sq metre accuracy.