Anyone had an issue where the air con blows red hot no matter what the climate control temperature is set to?

Yes. It's a relatively common fault. It's the overhead control panel (the unit with the light switches on it). If you give it a gentle tap with the palm of the hand, it should start working again - however, Mercedes will need to replace the unit. The good news is that it's a 10 minute job. The bad news is that the part was (not sure if it still is) on back order ......

I’m glad I’m not going crazy. Just had this happen over the last few days. I’ll try the palm tap on the roof controls and see how I go.

How hard should I tap?

No more than a swift upward palm slap :-) hard to describe forces in written form!