I created a new custom machine definition but ran into all sorts of problems with that, still not sure why but I got it to the stage where I could choose my custom machine type but after that I couldn't get any further, having to delete the machine definition in terminal before Cura would successfully launch.
Anyway, I've not given up on it as I think there is room for more fine tuning available by using a custom machine... if only I can get it to work !
In the meantime, I made the setting suggested above visible and, finally I see where it was getting those strange temperatures from ! Having set them all to the same temperature and checked the resulting code all the temperature strangeness has gone
I will have a play with them further to fine tune them but so far it's looking good, thank you !