I have read many posts about this problem and the answer is usually
"replace your ball valve washer". I have had a look in my toilet and
I do not exactly which part is the ball valve, although I do know
which part of the mechanism causes the water hammer. Also the parts
that causes the water hammer are made from plastic and do not have an
existing washer in the assembly.

I get water hammer through my pipes just as the cistern is at the end
of its refill. The long arm attached to the float has what looks like
a plastic screw mounted on it at the opposite end to the float. As
the float rises this screw pushes a "piston" into a "cylinder" (a
valve??) which eventually stops water flowing into the tank. If I
flush my toilet and push the piston with light pressure I can
reproduce the water hammer effect. The fact that the flowrate into
the tank slows as the piston is pushed in makes the water hammer noise
last longer.

The piston and cylinder (valve?) are made from a white plastic. The
piston fits loosely but I cannot pull it out of the cylinder. Does
the washer go into the cylinder before the piston, preventing the
piston from vibrating as it is slowly closed? B&Q have a ball valve
diaphragm washer for
98pence(http://www.diy.com/bq/category/categ...ml?CATID=86735. Is
this what I need?