A retrospective amendment to the Electoral Act is about as constitutionally repugnant as you can get. The law was clear that Harry Duynhoven was no longer an MP, and a by-election was mandatory. Because a by-election would have been electorally inconvenient, Labour changed the law to avoid it.
The precedent this has set is that the Government can retrospectively amend the electoral act to avoid an election, because the timing is politically inconvenient.
One of the most cynical and appalling abuses of power of all time.