In the end I decided to replace the toilet cistern complete with a new syphon valve, because there were condensation issues with the old cistern.
I used a Dudley Tri-Shell cistern which came with a Dudley Turbo 88R syphon valve. However, I have the same problem the internal overflow comes into operation when the cistern is full more or less to the top. I contacted Dudley and they said a 7.5" syphon valve should be fitted. How do you measure a syphon valve?
The new valve is pictured below, is this a 7.5" or a 9.5"?