Did they tell you that they'd fix the car for £96 if the work wasn't covered by a goodwill payment? The impression I gained from your OP was that they meant that the goodwill offer may not include a software upgrade required as part of the fix. I think you read into this that if MB wouldn't pay for the work itself, then the dealership would, but that's just wishful thinking.
So your tailoring comparison is a little skewed. However, if I was told that I would be charged for half and hour's labour even if they spent that time looking to repair them but were not able to do so, they I would say fair's fair - I wouldn't expect them to work for nothing. Having said that, I've been blessed with a tailor that seems to love doing small jobs for nothing, even when I'm fully expecting to pay.