Unless anyone else has some brainwaves it looks like I'll be moving the
cistern.

Don't know if this would work or not - how about heating up the flush
pipe with a heat gun / over a gas cooker flame and bending it to the
required "S" shape and allowing to cool?

Not sure what plastic these pipes are made of mind. IIRC it must be a
thermoplastic rather than a thermoset. They are quite cheap even from
B&Q so if you balls a few up, no major tragedy.

Another bodger's idea I'm afraid.