If you're struggling with the usual stuff, I'd use an acid based alloy cleaner. But wear eye and hand protection and follow the instructions to the letter. Worked for me on a set of VW Golf GTi calipers caked in crap
i've been using Meguiars "Ultimate All Wheel" cleaner for a few years and find it really effective.
The forerunner of the Mini .. the A35 .. lots of similarities except rear wheel drive. My engine contains a lot of Mini parts now and the bodyshell some Metro brake parts!