OK not that this has generated any interest - but maybe someone will come across it in the future;
The thermal switch is set to close, activating the over temp alarm
at 65C, and only resets (opens) again at 45C. Disassembling the raw water cooling
elbow I found some blockage here, have cleaned it out and reassembled it to the exhaust elbow.
Warm at idle and up to 2100rpm, it doesn't seem to go over 35C now, but no idea what the same point was before so I don't no if I have done any good. Waiting on a sea trial to know for sure..
If that doesn't do it i guess the next thing is to disassemble the heat exchanger
and clean it.