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By-elections

Douglas Carswell left UKIP to become an independant on 25th March 2017 and, much to UKIP's annoyance, did not trigger a by-election.

Wikipedia has an excellent page on these which is recycled below.

Switches of seats between parties are colour-coded.

Constituency date winner swing previously reason
Manchester Gorton 4 May 2017 cancelled   Gerald Kaufman (Lab) died age 86
Stoke on Trent Central 23 Feb 2017 Gareth Snell (Lab) -2.1% Tristram Hunt (Lab) new job at V&A
Copeland 23 Feb 2017 Trudy Harrison (Con) +6.7% Jamie Reed (Lab) new job at Sellafield
Sleaford and North Hykeham 8 Dec 2016 Caroline Johnson (Con) -0.3% Stephem Phillips (Con) irreconcilable differences
Richmond Park 1 Dec 2016 Sarah Olney (LibDem) +21.73% Zac Goldsmith (Con) 3rd Heathrow runway
Witney 20 Oct 2016 Robert Courts (Con) -19.3% David Cameron (Con) flounced off
Batley and Spen 20 Oct 2016 Tracy Brabin (Lab) n/a Jo Cox (Lab) murdered
Tooting 16 Jun 2016 Rosina Allin-Khan (Lab) +7.3% Sadiq Khan (Lab) became London mayor
Ogmore 5 May 2016 Christopher Elmore (Lab) -0.8% Huw Irranca-Davies (Lab) to stand for the Welsh Assembly
Sheffield Brightside & Hillsborough 5 May 2016 Gill Furniss (Lab) +4.1% Harry Harpham (Lab) death, cancer
Oldham West & Royton 3 Dec 2015 Jim McMahon (Lab) +2.3% Michael Meacher (Lab) death

The conservatives, libdems and UKIP did not field a candidate because of the circumstances of the vacancy.
A swing is calculated by comparing the percentage of the vote in a particular election to the percentage of the vote belonging to the same party or candidate at the previous election. Wikipedia.

Sources
Wikipedia