Pulled the N205 valve out last weekend, sprayed it with degreaser and then soaked it in petrol for a few hours. Everything looked clean and in good working order upon inspection. There was no debris etc in the valve that was hindering its movement.

Re-fitted the N205, no issues. Car still rattles on startup so the valve had nothing to do with it.

If any of the mechanics on here wants the special tool, let me know.