Now then all.
After half a dozen posts and plenty of responses I now have a working cast iron cistern with an overflow that is just about up to the job.
A big thanks to
ChrisJP
Stuckinarut
Exedon
Wayners
opps
Madrab
jj4091
Nige F
Hugh Jaleak
foxhole
oldbuffer
For all your help, non of you had to...