Of 650 MPs, Sinn Fein have 7 who never take their seats.
So parliament has 643 sitting MPs
Of those 643, the Speaker sits as an independent, and he never votes except to break a tie, when he votes for the status quo.
Of 642 voting MPs, the Tories and DUP have 322, and the Opposition have 319.
But there are three deputy speakers who don't vote as well. One is Tory, and two are opposition. Which leaves 321 - 317.
If the Lib Dems win Brecon and Radnor today that will add one MP to the opposition, making the tally 321 - 318.
A Tory-DUP operating majority of 3.
So if the Lib Dems win today's by election, we make legitimate claim to know that there will be a general election this autumn.