I'm pretty new at this too Duncan. But here's the things I found with my set up.
By the looks of the trails, not guiding in DEC.
Polar alignment not close enough. If you can get reasonably round stars on a 2 min or better unguided exposure, its ok.
Your guiding on a hot pixel (done that myself, as have experienced others who shall remain nameless
)
Too much backlash in the drive train, probably the DEC.
Step settings in PHD too big, inclined to make it "hunt" a bit.
From your description probably not this, but do the calibration after slewing to the object and re-do it (force calibration in the "brain" menu ) when you change to a new target.
Bill