@MikeInWimbledon Not a silly question at all! You are correct, the normal 3 pin plug will charge it somewhere between 2 and 3 hours. So your plan would work.
There are two reasons I went for the 7kw charger. The first is that it is tethered so I don't need to open the boot and pull out the cable or go into the house to get the cable. Sounds lazy I know but the idea of making it as easy as possible for myself means that I am more likely to consistently do it!! The second is future proofing at a low cost. I would have needed to have a 3 pin socket installed outside which would have cost me 120 euro. I got the 7kw charger installed for 950 euro and got 600 refunded through the ev charger grant we have here. So for 230 euro more than the cost of the 3 pin socket I have the Zappi tethered charger which will hopefully set me up nicely as am likely to go full EV or at least PHEV with a bigger batter next time.
For what you need it for, it sounds like the C350e would be a very good fit for you in that I take it from previous posts you want to stick with Merc and don't want to go full EV at this point?