In an earlier contribution to this discussion,
John Smith wrote:


I seem to be caught in a dilemma whereas if I do not tighten the nut
on the valve where it enters the cistern I get a slight leak.
However, if I tightend it enough to stop the leak the valve
turns/twists inside the cistern and I have the ball-cock issue. The
plastic is too fragile to grip whilst tightening and so it is night
impossible to stop it moving. It seems that the gap between the
siphon and the valve is so small that one or two milimeters at most
is the difference between success and failure.

Why not install it slightly clockwise (in top view) of where it needs to end
up - such that when you tighten the nut, it moves into the correct position?
It may require a bit of experimentation to get it just right!
--
Cheers,
Roger
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