I think I've finally got this sorted, today I had the thermostat (230890) and the fan resistor (263140) replaced. The thermostat seemed to be sticking open and not getting the car upto temperature and the fan resistor had corroded away to pretty much nothing so was obviously not functioning. The car now gets upto temp (about 90) pretty quickly and I've tried a few times starting the car after stopping for a very short period and it seems fine
So although P0128 points at the temp sensor it's more likely to be the thermostat or resistor, this is something one of the chief techs at Maserati UK has also informed me
Hope this helps anyone else who has the same problem
well took it for a service and discussed with them about your solution, suggestion from them instead paying out for these parts (~£100) was to have a software update, which has a pre-req of the new temp sensor which we've both had.
So far so good (only filed up once), Will see how this works out and report back
thank you - after what you said had fixed yours, I wasn't sure an update on mine would make any difference - yep very strange. also like you experienced - mine also seems to drive a little better