This sounds like the Twyfords Nocturn toilet and the plastic part is in two pieces, one that connects with bolts to the oval plate and seals with the doughnut onto the base of the cistern(horizontal plastic face) while the other (vertical face) connects to the back of the toilet pan. Also with two bolts. The second piece of plastic is a tube with three openings and fits into the back of the pan. This distributes water round the bowl. It pushes on to the other plastic part and there is a soft seal, hollow cylinder in shap, that keeps it water tight. I am told it is called a 'nocturn accelerator' is in white plastic and now as rare as hen's teeth! Good luck finding one.
Ian
August 2009