Hi there,
I have to crank the flush lever several times before my toilet flushes. A neighbour suggested that the "plastic flap valve" inside te siphon needs replcing.
Can anyone confirm if this is the probable cause and if so how would I go about doing this?
Any help appreciated.

speedball1

It's probably not the flapper that needs to be replace but the linkage adjustment between the flapper and the flush arm. Let me show you some adjustments that you can check in addition to the flapper.
The water level in the tank wants to be 1/2" below the white overflow tube when the tank's filled and the ballcock shuts off. If you have to make adjustments to raise or lower the level here's how to go about it.
Grip the float ball rod between the ballcock and the float ball. DO NOT GRIP THE FLOATBALL ITSELF!! The threads will break under pressure. To increase the tank level bend the rod up. To lower the tank level bend the rod down. Keep "tweeking" the rod untill the ballcock shuts off 1/2" below the top of the white over flow tube. Since we have the lid off let's check the rest of the adjustments.
Feel the flat side of the flapper. It should have no wavy edges. If it does replace it.
With the flapper seated the linkage,(the chain between the flapper and the flush arm) wants to have 1/4" of play. Too much play and the flapper doesn't get pulled back enough giving you a short flush. Too tight and it lets water seep past the flapper. I adjust the chain by the link and if it needs fine tuning, I bend the flush lever rod a bit untill I have the desired play in the linkage. One more thing about a flapper. you will see where the old flapper hooks onto the base of the white overflow tube. On a older type with no hooks the flapper has a ring that slips down over the overflow tube to the seat. If your tank has hooks, take a sharp knife or scissors and cut the neoprene ring off on the marks provided and hook the flapper on the hooks. Leaving the ring on will interfere with the flush. Now check to see that the little white tube running from the ballcock to the overflow tube is connected and dispenses water while the ballcock's filling. It increases the bowl level. Hope this helps Tom