My local mower shop is now open after the break and I have bought a replacement for the rubber plug.
I opened up the carburetor and found that I have installed the diaphragm the wrong way (upside down). I correct the error and the mower now starts and runs.
I do not know why it will not start before as when I cleaned the carburetor, it was not dirty and nothing was blocked.
Even though it is now running, I don't think it is running as smoothly as it can, especially at low rev. However, I must add that it has always been like this for a long time. What can I do to improve it?