Given the reason for the by-election being called it is frankly astonishing that Labour won the seat again especially with an increased margin over the second placed candidate.

Not really for those prepared to dig a little
https://www.peterborough.gov.uk/council/about-peterborough/census-2011/

The size of the majority is irrelevant. If you don't understand how representative democracy works I suggest that you go back and re-read Burke.

So pray tell, why do lefties use mathermagic to constantly dispute the outcome of the brexit referendum mandate to leave the EU?

As Spike says:17.4million Leave voters – the largest mandate for anything in British political history