Quite intriguing. For my 2012 C350, I am presently paying £44 a month for a three-year plan. This plan will cover just one extra service. At the beginning of 2013, the monthly cost was £27, so I decided to get a plan. It accepted payments all the way through 2016. After that, I had a £27.50/month plan for two years, and then a £36/month plan for four years. For as long as there has been an option, I have always gone with the longest plan. thought it was worth it the whole time.
Whenever I needed an extension, I had to ask for it; neither my dealership nor MB UK ever suggested it. You can see that the price has gone up from £27 to £44 each month.
The "replacement vehicle whilst car is in workshop" provision is no longer part of the plan, according to the MB website. There was a little fee "to cover insurance" for this, but up until my most recent visit, it was always free. It is likely to be dealership or group specific.