How significant are Burnham’s remarks on Kremlin interference in Brexit vote?

Andy Burnham unexpectedly reopened an old Brexit debate when he said he had been told after becoming prime minister that Russia had interfered in the 2016 EU referendum. It was a contentious issue in the second half of the previous decade, but one that had largely been forgotten about as the UK finally adjusted to the reality of leaving the EU. Now Burnham’s comments have given the topic a fresh impetus.