Hi Wicked Wizard,
The bellows is OK. I think I have found out what the problem was by pure accident! I have put the green plastic tube and clear plastic tube connecting to the cistern back on the opposite way round and the cistern flushes perfectly and dosn't...
I have two problems with my Thomas Dudley Vantage Dual Flow Concealed Cistern. The first problem is that it requires several sharp presses of the flush button for the flush to work. The second problem is that the cistern overflows into the toilet bowl after refilling for a few minutes then stops...