Thanks folks. I had seen both 1947 and 1960 given: looks like the latter.

In the 'Chapman' book of HH's poetry, titled funnily enough 'Freedom Come All Ye', (1999) the meaning of rottans is given as 'rats'. Gar definitely means 'make': my wife's grandfather was still using the word in the 1960, in Angus. So, I take 'gar oor rottans' to mean 'make our contemptible political leaders'.