Screw on seal/gasket for upper arm HDS 555 M?

On my Hoover dishwasher HDS 555 M there is a seal that screws to the upper spray arm delivery tube, inside the diswasher at the back. It Seals around the "spout" that delivers the water to the upper spray arm, when the top basket is pushed in. I have been trying in vain to but a spare part. If anyone knows the name of this part or better the Hoover part number, I would be grateful. Thanks