Another thing to check is to turn your old diaphragm over to look at the bottom. Size 32mm (1-1/4"). All red, red & white or all white, this indicates year of manufacture and has no effect of the function of the washer.
Armitage Shanks Contour 21 Basin Overflow No Chain Hole 500mm 1 Tap Hole
Armitage Shanks Sandringham 21 Wall Hung Basin 550mm Wide - 1 Tap Hole
Armitage Shanks Sandringham 21 Washbasin 55 cm 1 Tap Hole with Overflow E894901
Armitage Shanks Contour 21 Plus Basin Back Outlet 400mm 1 RH Tap Hole
Armitage Shanks Sandringham 21 Corner Basin & Full Pedestal 480mm W - 1 TH
Armitage Shanks Portman 21 Wall Hung Basin, Overflow 500mm, 1 Tap Hole S231101
