Any question answered...

Why does my left crank keep coming loose?

It did it last week and I had to limp to the nearest bike shop to tighten it up. Then I set off out today and noticed it was coming loose again. How can the threads on a brand new BB/hardly used crankset be fucked already? Am I doing it wrong?

Sometimes you get a new BB but don't get new crank bolts as well. Alloy threads on old bolts can be slightly worn.

I've got better results from getting a new set of (allen type) bolts with every new BB and then only installed them after I've tightened properly with the old school steel bolts (socket type). Seems to work OK.

More likely for the thread on the softer alloy bolt to be screwed that the one in the harder spindle.