Gareth the story re Thomas Atkins is true according to Wellingtons various biographers - and if memory serves me correctly the original pay-book is on display at Stratfield-Saye, in Hampshire.

The poem of Kipling's appears to be set at a time when the British army marched in column of fours, not threes (each row of the column being four men, each line being the rank). The change from fours to threes was made to make them less susceptible to straffing by aircraft in the First World War.