Our en-suite loo has stopped flushing and I'm guessing it is the seal in the syphon, though with the flush handle disconnected I can pull hard enough to make it flush?

However, I'm struggling to get the close-coupling cistern wingnuts off (it was installed in 1991!)

I have WD40 on them for now, but with a wall on one side, the bath on the other and a pair of rusty wingnuts to deal with that are facing the wrong way for a wrench to get a grip on, I'm hoping someone can tell me a secret tip to get these off or some other way to remove and check the syphon because i really don't want to refit this bathroom just yet.