Went to the shops. The car body measured 2C as did the 4 disc brakes on the car. Started the car, the dash thermometer confirmed 2 degrees aswell, drove to the end of the street, didn't have to yield to anyone so proceeded on the main road and got up to 50 mph. A few hundred yards up the roads is a collection point for glass where I can pull over so I braked and got on there. No traffic so could do it in one smooth motion.
All 4 brakes registered between 10 and 11C after the stop from 50 mph. Proceeded on my way but took a very scenic route, over 20 miles of rural roads, lots of turns and intersection, and a few miles of dual carriage. I braked 4 times from 50 to 20 on a longer straight section, for no other reason than to heat up the brakes. Everywhere I could I braked a bit more than I normally would and I approached a bit faster as I normally would.
When I got to the shop, the front brakes were 51C, the rears 32C. The front wheels measured 18C and the rears 15C. Touched the rear discs and they were warm, but not hot to the touch. Shopping took about 15 minutes and when I got back out, the brakes and wheels were fairly even in temperature, around 20C. Took the short way home (less than 3 miles) and drove as I normally would. More traffic now so had no choice in the matter.
When I got home the brakes were 28 degrees front and 35 degrees rear. Warm, but not hot. Could definitely hold on as long as I wanted. This included 4 sets of traffic lights where I had to stop. But the majority of travel was uphill so the brakes have light duty and the rears actually can work to their full potential.
The car weighs 1500 kg +- 5 kg with a full tank of gas (like today) and I weigh 90 kg. It's a medium weight vehicle and has been weighed 2 weeks ago at the recycling centre (when entering, and when leaving)
Front brakes are 330 x 28mm , rears are 278 x 12mm. Good size, but not the biggest either.