How can I sort out the flushing of my old Royal Doulton toilet?

The siphonic action on my old Royal Doulton toilet (probably installed in the late 1960s) has stopped working and the bowl overfills before slowly draining away. Disconnecting the cisterm I found that the donut washer (internal hole diameter 55mm) has disintegrated and needs replacing. Is it possible to get a replacement rubber seal and, if so, from where? I have tried searching the internet but so far been unsuccessful in finding a supplier. Your help would be much appreciated.