Well three years on after re-plumbing new electric toliet and holding tank finally she gave me the ****s! After back flushing with blue expensive useless chemicals the tank remains full when sea cock is open.

Today I removed the elbow I installed and as expected it was chocka block with stuff with just only a small hose size tunnel!

I was was going to order a new black box tank as I suspected it may of been full of a hard sludge but now the elbow is cleaned out all is running well. The suction hose I used which i was told I shouldn't ..had very little build up due to being a shiny rubber .....it was the pvc elbow that caused all the probs. grrrr what a poo day.