how can i replace a siphon unit on a close coupled toilet?

My Derwent Macdee close coupled toilet will not flush, i have checked the obvious ie linkages and come to the opinion that the fault lies in within the siphon unit. Can you describe in detail of the sequences necessary in order to replace the unit and is it necessary to replace the doughnut washer at the same time as the toilet is only 5 years since fitment?