Mini Cabrio Diesel Overheating

Sounds a lot like what happened to my Mini, though mine's not a diesel. Overheating and taking ages to warm up points to the thermostat being stuck open. The map thermostat, like you said, controls how much coolant flows to the engine. When it fails, it usually stays open, causing those symptoms. Have you checked for any coolant leak around the thermostat housing or near the temperature sensor? A small leak can also contribute to these issues.

I haven't noticed any coolant leak specifically, but I'll keep an eye out. Where exactly is the thermostat housing and temperature sensor located in relation to the engine? Is it something I can easily spot, or is it buried deep?

The thermostat housing is typically connected to the engine block, where the upper radiator hose connects. The temperature sensor is usually right next to it, screwed into the housing. But without specialized knowledge, I wouldn't poke around too much. Given the overheating issue, it's best to get it checked by someone qualified before you cause further damage. Engine overheating is a serious problem and might damage other components of the cooling system.

Thanks for the input. I had it checked out and you were spot on. It was indeed the thermostat. Repair set me back around 350€, as expected.