The biggest takeaway from the Brecon result is not that there is no mandate for a no deal Brexit - we knew that already. It is that Labour are a busted flush with Corbyn in charge. Nobody is going to vote for them.
I'm sure it's all the fault of Peter Mandelson and Alistair Campbell though.
Although you do need to take into account that Labour haven't held this seat for decades - it's mostly been Lib Dem. That said, the next GE could be interesting - I'm going to predict a 4 way tie between Tories, Labour, Lib Dems and Brexit Party. How they form a government out of that is anyone's guess !