You may know that Gridserve took over Ecotricity see. Perhaps the ones you are referring to were older Ecotricity ones and the map has not been updated. Just guessing!. Purely by chance, I saw 2 Gridserve charging points just off the A1 near Peterborough yesterday when I was using Ionity at the same location.

Complete guide to the Gridserve (formerly Ecotricity) Electric Highway charging network | DrivingElectric
Need to charge your electric vehicle on the Gridserve Electric Highway network? Here's how it works, how much it costs and how to pay

It may be that Sat Nav is out of date. Are you using the Mercedes me app?