So, some progress, but still not enough!
I found the silicone rubber sheet and used the laser. So that's washers sorted.
The design here has two holes in the bottom of the cistern, not the side. So the fill comes in that way, the overflow also leaves that way, via a 1" diameter plastic standpipe with a threaded end. I can see no replacement for this, nor anything to connect to it.
The problems now come down to poor past installation. The inlet pipe is short vertical copper (thankfully with an isolator!) and has no flex in its mountings. So attaching it requires the fill valve to be installed, then the cistern lowered onto it. There's just no way to install the cistern and then connect the pipe to it.
The overflow is a bodge-tastic chain of unglued overflow pipe, running up to a large L bend that goes onto the cistern standpipe. This has a female threaded end which fits onto the standpipe (a plastic fitting seemingly now unknown to modern plumbing). This has broken enough times in the past that it's now largely a blob of car body filler! Sadly it's also irreplacable. This pipe is long enough that it hooks under the rear of the pan (so can't be installed before installing the cistern). The only way to install it seems to be to drain the cistern completely, then unscrew the standpipe and seal washer from the inside, and unscrew it from the overflow pipe. (water gushes everywhere). Re-assembly involves fitting the cistern to the pan, screwing the standpipe into the overflow, then tightening up the clamping nut blind and hoping that it feels like sealing. Screwing the standpipe too far into the overflow will neatly split the brittle fitting in half at the end of the blind thread (explaining the layers of historic body filler).
Now, with it all together, I find that the original fault is still evident and it's still leaking between the cistern and the siphon, so it all has to come out again. FML...