@Chris Mc I would get the brakes stripped down and cleaned, make sure the calipers are not seized causing overheating on the disks
Then change disks and pads. But the caliper service is important.
Another way is if you have one of those in the ear thermometer for kids, i set mine for body temp, then scan the brakes to see which one is heating up the most. If not, then I do the spit test. Gross yes, but works as you can see which one sizzles the most lol
Or touch the wheel hub for any heat differences. if both radiate lots of heat then a service on them is needed.
Also sounds like calipers have not been retracting from those meaty pads and the overheating on motorways.
I recently changed all 4 pad sets and drove around europe.. the wheels on all 4 sides were really black when we go back, never been black like that on our previous runs, so it shows how much bedding in they need.