£50 to fit a siphon ?? thats expensive, the job should only take 30-45mins.
Ive done many of these myself and its really simple.
If there isnt a shut off valve dont panic easy to get around.
Here's what i would do.
1, Remove(unscrew) the float off the ballcock to give yourself a bit of space
2, Wedge a long screwdriver under the ballcock arm to hold it in the up position and it should cutoff the water
3, Flush the cistern to empty the system, and make sure no water is filling the system back up(as in No2)
4, Place a large bath towel or a bucket(if 1 will fit) under the cistern where the pipe is what goes to your lou. This is to catch any excess water out of cistern
5, Unscrew the pipe off the bottom of the cistern and remove the siphon
6, Replace new siphon using PTFE tape on the screw joints, and connect up the handle pull
7, Once your saticfied its all connected up, remove the screw driver from under the ballcock arm, but still holding it in the up position slowly lower it to allow the water to fill up the cistern slighly, and check for leaks around the screw joint under the cistern, if alls ok, then refit the float to the arm, and let the system fill back up, and re check for leaks again
Hope this helps
and noway would i pay £50