Yes of course you're right that a syphon overflow can be used- in principle!
Not in practice, though, in this case! A syphon will overflow into the pan if the water level rises above the bottom of the highest point of the syphon tube. But our cistern is a Dudley "Elite".
So I'm still looking...
Thanks to you both, but I did mention that our downstairs WC consists of a low-level cistern and (separate) pan, and that it is not feasible to change this.
I'm still looking for a way of connecting overflow pipe to the inside section of the flange fitted to the cistern.
In the modern...
The WC is in a downstairs bathroom. on an outside-type house wall that has my workshop on the other side. There is a outdoors-type gully in the concrete floor of the workshop for the wastes from the bathroom. The end of the overflow pipe from the WC cistern is in same place, and is visible from...