I know this sounds like a Bad Idea, but I shall explain.

I am renovating my flat with the help of eBay (99p for a brand new
double-ended bath anyone?) and acquired a Porcher toilet and cistern
fot GBP8.50 over the weekend. I am installing a flap-flush mechanism
that has its own, built-in overflow, so I need to seal the overflow
that is in the cistern already.

I can't use it as the inlet as there is already a bottom inlet. I
can't actually plumb it as an overflow because the overflow on the
existing toilet is *higher* than the overflow level (plus I would
rather do away with it anyway).

So how can I seal it?

I don't mind the internal pipe remaining there as it will be unseen
although all things being equal, I would prefer to be rid of it.

I bought a push-fit stop-end, but it is bigger than a 22mm stop
end. The screw thread on the outside seems to be 25mm diameter and the
pipe inside the cistern appears to be ~26mm external diameter.

Is it likely that I am mis-measuring and the pipe is in fact 25mm? If
so, has anyone seen a plastic push-fit stop end in that size, because
I haven't.

Or has anyone seen anything that looks like it is a blanking
thing designed to stop a hole in a cistern?

Or has anyone seen a stop-end for a 25mm screw-thread fitting?

Thanks for any suggestions.

Pete