Nope, no balance shafts!To anyone reading; are there any balance shafts within the 1200 engine?
With the sort of milage quoted at the start, any bearings in or around that kind of area may be suspect.
With the vibration "driving itself out", as with a steering wheel wobble on a car, I would summise something like shaft/bearing.
Just a thought. Nasty, I know; but possible.