BTW although I'm a fan of present tense I
think that opening highlights some of the challenges!
^^^
Have I screwed up?
Thanks for the read. What happened to the witchey wife?

No, I don't think you screwed up. But you go from what is happening (you leave) to what will happen in the future (the irony will not be lost). So grammatically correct, but for me at least, slightly jarring. How do you know what will happen as it hasn't happened yet? The irony might not occur to you.
^^^ Okay. As long as it's correct that's fine. I'm a big fan of Kate Atkinson and she jumps all over the place and I love it! 
The witchey wifie tried to cast a spell on me, but I grabbed a mirror and deflected it back at her. With a loud fizz, she turned into a bat and flew off into the night.
WOW! What a lucky escape.