Having a water meter, I wanted to reduce my consumption with a dual flush. This is cheap (£20) and easy to do for for separate cisterns but for close-coupled cisterns with rusted-on connectors in a confined space, it's not a pleasant job.
There was no help on the internet on how to convert a singe flush siphon to dual flush so I made my own and describe it here for comment.
It's quick for someone with simple diy skills. It cost virtually nothing and could can save £20/person/year on metered water bills. The device consists of a ~200mm of rubber hose, the same amount of string and a nut to weigh the hose at one end (see photo for illustration)
Drill a hole near the top of the plastic cistern bell, just slightly smaller than the hose. Thread about 20mm of hose into the hole which should be a tight fit so it doesn't fall out. Fit a tight fitting nut to the other end of the hose to act as a weight. Tie the string between the nut and the hole at the top of the piston rod.
Now when you flush, the hose is lifted up from the bottom through half the depth of water and if the toilet handle is kept down, water stops flowing at half the water height as air enters the siphon.
Alternatively if the toilet is flushed normally without holding the handle down, the hose drops to the bottom and a full flush is obtained.
Note: there could be safety issues with drills close to water, so best to drain the cistern first and tie up the ball valve to keep the water out. Sometimes there is a plugged hole in the siphon (used for setting the initial flush volume) which can be used, making the job easier (may need to pack PTFE tape around the hose to make a good seal). Some siphons can easily be extracted from the cistern (designed for easy diaphragm changes) making it easier and safer to drill the hole.
Note also some siphons are dual flush but have been deactivated by a previous owner. Worth checking first.