I quit using Midas when they wouldn't put in my windshield wiper assembly (lift out and connect a few fiddly bits) in my wife's car. They wouldn't order it for me either.

I follow the car's recommendation on servicing and use the MB standard, which allows for several different weights.

I have a few shops I'm OK taking my Chrysler to, but if I were in the UK I would definitely look for an independent MB shop comfortable with the first generation SLK cars, then lift the bonnet and ask him if he was down with it.