There have only been five Westminster by-elections which have been contested by the main parties since the Brexit referendum and the average party changes are shown in the chart above.
As can be seen there have been much bigger movements in constituencies which voted Remain than those which voted Leave. In the former, both in CON held seats seats, the LDs did particularly well picking up one gain with the Tory vote sharply down.
What we haven’t had is a by-election in a LAB seat which voted Remain as is the case in Lewisham East so we are into new territory.
What is important is that Westminster by-elections can develop dynamics which are very different from the overall political picture and comparisons with local results or what happened at the previous general election are not necessarily relevant.
I don’t buy Pulpstar’s analysis of Lewisham because the scale of the LD operation is likely to be at Witney and Richmond levels and will be far greater than that of the Tories.