I'd not appreciated before, why VCR mode change motor and gearboxes always have a small drive band from the motor pulley. These Arcam gearboxes are cogs right through. Alright at the outset but any dirt, wear etc that increases the back torque then there comes a time when the motor cannot start to spin because there is no give in the gear train.
Final repair attempt, the (slightly cut -down to let in band) casing and spindle from a small motor, with magnets removed, to replace the original. Then a rubber drive band around the rotor, to the motor, repositioned wherever is suitable. Should provide that bit of start-up give and also an extra 5:1 mechanical advantage.