As you will have to take the syphon out it's not worth repair as it seems the hook end that fits the arm has broken off and as you have to remove the syphon you might just as well replace the syphon. But its up to you of course.You might get away with making another hook from a wire coat hanger but it will eventually go rust as the originals are brass.
Procedure:
Start by turning off the water at the cistern feed stopvalve or if there isn't one you can tie the ball valve to a piece of wood across the cistern but keep the wood clear of the syphon no matter if you can't you'll see later.
Bail out the water completly. Lay rags or towel under cistern to catch any water in flush tube. From under the cistern, Undo the flush tube nut and remove the flush tube from cistern. Undo the syphon retaining nut remove the syphon. [if the wood is in the way move it holding on to the ball arm to keep it closed.]
Lift the syphon out of the cistern obtain another one, if slimline tell supplier, if standard cistern most will fit.
Revers procedure installing new one.
,
Nobby, February 2009