Yeah I would not jam the belts in the door and pressure wash them!
I purchased a ÂŁ30 steam cleaner, put Autobrite APC in the detergent tank, set the mix ratio to max and got to work on the belts. Worked a treat! Steamed them, then agitated the belts with a soft nail brush, wiped clean with a microfibre cloth, then steamed again with no APC mix, just steam and wiped again with a clean microfibre - belts looks great now! Advantage of the steam cleaner is I can do the rear belts on a 3-door car.