David Davies' decision to resign is not a challenge to the Labour Party on the question of 42 days. If he wins or loses it won't make a difference to the Labour Government.
His challenge is to his own party.
Most Tories who voted against 42 days would be happy with the policy had a Tory Government proposed it. Few Tories would want to stand on a manifesto that promised to reverse the law (if it is passed).
What Mr Davies' decision does is to throw down the gauntlet to those in his own opposition rich but policy poor party to say we will reverse this law if elected. It is a clarion call to Tories to make a stand on an important issue, rather than opposing for opposition's sake.
The Tories will, of course, make the stand in order to win the by-election. With no Labour, Lib Dem, UKIP or BNP candidate opposing him the chance that the Eng Dems, or worse, the beauty queens, might give him a run for his money would be unthinkable!
But the main question is will the Tories pick up the DD gauntlet and become a party that proposes rather than just opposes? And if it does will that enhance or harm its electoral chances?