Last question. What is this thing? It came with the under sink pipes and tubes. I reckon it is something to block water coming back into the dishwasher and goes on the connection. But I'm not sure.
Fittings often come with all sorts of gubbins like that. Have often frowned at the detritous the plumber leaves in the plastic bag after fitting something. But it all works OK so who knows? Probably to cover several scenarios.
Finally done. Small leak due to wonky pipe i can solve with some silicone sealant and then the dishwasher outlet has a tiny drip I cannot for the life of me solve. Fuck it