BigWallop coughed up some electrons that declared:

The down pipe from the cistern should allow for cutting to the position of
the bowl. If the down pipe has been cut before, it may need lengthend on
the horizontal to match up with the pan in your new setting.

I was always asked, many years ago, when these were the normal flush
loo's,
to replace the older cast pipes for new plastic pipe. The only way to do
it
then, was to buy the length of pipe and a solvent weld natural bend. Not
an
elbow. A swept bend. So when you're out buying the new pipe, ask the
merchant for a "90 degree Swept Bend" and a new rubber flush inlet gasket
to
suit the size of the new pipework. (32mm or 40mm)

Hi,

Slight misunderstanding... There's enough pipe (it's metal).

What's happening is the pipe isn't sitting parallel to the wall - it's
trying to push back into the wall. Something isn't straight (no surprise).

By packing the base of the cistern out, I'm changing the angle to allow the
pipe to swing forward without straining the pipe-flush valve coupling.

Ta

Tim