Toilet Cistern

Replace the diaphram on the bottom with a new one.

It is likely that the polythene diaphragm in the siphon unit has disintegrated, resulting in lack of sufficient suction to siphon water down to the toilet bowl. You may be able to replace the diaphragm if you can find one to fit. To do that you will need to remove the siphon unit after turning the water supply to the cistern off and draining the cistern of water. If you can't or don't want to replace the diaphragm you can just replace the whole siphon unit. A new unit is only about a fiver. Take the old one with you to get a similar size/type of siphon unit. New siphon units can leak from the nut/thread under the cistern; I use clear silicone sealant to prevent this.

. You may be able to replace the diaphragm if you can find

A tip from this group when I had the same problem was to remove diaphragm and replace with a piece cut from one of those clear (usually coloured) plastic document wallets/folders. I did this with two toilets and they are working as new.

My units were designed so that you could get access to remove the syphon easily, without having to do anything too techical. After that it was dead easy to replace the diaphragm as above.