Thanks Nigel – I’ve now accessed the table but there are only two entries with module_name=html — the main block id871 (location=main) and another id17 (location=side) which seems to correspond with my existing secondary HTML block, which was positioned on the side. The temporary block I was trying to add was in the ‘main’ position, directly under the original main HTML block.
From this I conclude that an additional block has not been created but that the process somehow hung after those two errors were triggered. The Home Page still displays only the original main HTML block and none of the other seven blocks (four main, four side which were there before).