We were somewhere around Barstow, on the edge of the desert, when the
drugs began to take hold. I remember "the_constructor"
saying something like:

Is the same cistern used for both the low level toilet and close coupled
toilet (the latter being fixed with a clamp plate underneath), because if it
is, I will only need to replace the toilet pan.

Usually, yes. If you remove the close-coupling kit (the plate with the
slotted holes) and simply put a pair of L-brackets on the wall, the
cistern will sit on them happily, combined with the screws into the wall
to actually secure it.

Most close-coupled toilets use an ordinary cistern (part of the
manufacturers' range that matches the style of the toilet pan) that is
made to be a close-coupled one by the kit.
--

Dave