I tried the warm clothing route, the problem is a front window that never clears of condensation! The £90 is money well spent. I bought an icarsoft reader for just a bit more money and can now read fault codes myself but didn't fancy fixing this issue.Luckily I swapped main dealers after the first one left me a voice message saying that the time and date I booked on their system was not actually available and if I still wanted a service to ring them back. I can understand errors - the time slot and date not being available but poor customer service to just leave a voice message and throw the ball back in my court I thought, so I've gone elsewhere (still main dealer). They will, for £90 (an hours work) investigate and think they should be able to find the issue in that time (probably by plugging in the OBD as you say). I will find out on 1st Feb - if it's £600 to fix like yours I'll be ok with that, anymore than that kind of price and I'll not bother and just wear thermal underwear in the winter !!
Just curious but what year is your SLK? Mine is 2012 and the one the dealer just repaired was also 2012.. this might be an emerging problem for R172s in colder/damp climates.