The newish to me policy of siteing the thermostat in coolant return seems to cause lots of anomalies. As I understand it, it aids quick warm up with emission reduction benefits and controls engine temp more accurately given modern bhp/litre engine outputs. It's certainly more complex though and maybe explains why Peter J found his engine temp rising above normal in cold weather before settling down. Ditto for folk who have fitted larger radiators and found engine running hotter initially.
My nephew has a Ford Fiesta just out of warranty with the turbo one litre three cylinder petrol engine. It's now almost undriveable with low compression on one cylinder despite only 24,000 miles covered. The issue is known to Ford and caused by the complex cooling system consisting of two water pumps (one mechanical, one electric) two thermostats, pressure bypass valve, rad and fan plus oil cooler working as a three stage progressive system. Despite this, severe overheating has been noted with coolant temps up to 180°C being measured by third parties whilst the bottom of rad was cool! He is presently in negotiations with Ford who want him to first pay £1000 to have the head removed and checked