The timing might not help BJohnson
The Sheffield Hallam MP, Jared O’Mara, has made it clear that as soon as the Commons returns from its summer break he will step down thus triggering what could be an interesting by-election in Nick Clegg’s old seat.
Labour are already throwing everything at defending their great victory there at GE2017 while a full LD by-election operation is already in place. Assuming that the by-election is called almost immediately, then polling day could coincide with the crucial European Council meeting that starts on Thursday October 17th.
Given that the best you can get on the LDs re-taking the seat is 1/14 then a likely victory by the strongest pro-EU party would send a powerful message to Brussels at what could be a critical moment. This might not be helpful to Mr. Johnson assuming he is still PM then.
Given that Hallam voted strongly 66-34 in favour of Remain in June 2016 the LDs will, unlike Brecon, be making Brexit the central theme of their campaign. The LAB effort looks set to be about the Lib Dem role in the coalition.
Now all this, particularly on dates is speculative. But if O’Mara does as he says he will then it is hard to see the by election not taking place in mid or late October.