Solved! The bolts holding the seat to the floor, especially at the front, were loose.
Spline bit set from Halfords (you need the M10 spline), torque wrench from Screwfix - about £40 the pair.
Slide the seat all the way back, tighten front bolts.
Slide it all the way forward, tighten back bolts.
The Haynes Manual for the Mk5 doesn't give a torque setting for the seat bolts, but elsewhere on the internet suggested 40Nm, which didn't seem crazy.
Worked even better than I expected, no sign of jolting or rattling now