How to replace outlet washer on a Caroma back-to-wall dual flush toilet?

I need to replace the outlet washer on my Newport back-to-wall toilet. There is a copper pipe partially across the top of the cistern. I removed the inlet unit from the right but need to remove a threaded nylon tube which links the copper pipe with the inlet unit. Then I can release the unit underneath (by 4 clips) to get to the washer. It won't budge and I don't want to break the plastic. Any ideas and am I doing it right?