Update on my original post. Drove home after midnight on Saturday, mostly motorway and it was chilly. Felt like 11 / 12C but definitely under 15C. The gauge still showed over 90C so I think that is just 'normal'. Checked the next day and no problems with hoses or leaks but the water in the header tank was at minimum level so I topped it up. Thanks for all the comments which have been very helpful. Will be getting the ODB dongle anyway.
