Ideally the car needs to go on STAR, the Mercedes diagnostic reader to find the actual fault with your SRS. It could be one of the seat occupancy sensors failed but STAR can confirm exactly the problem. It is an MOT failure BTW as I see you have just bought the car.
Mercedes dealers aren't exactly cheap when it comes to diagnostics so using a Merc independent is the way to go. If you post your location someone can recommend where to take the car.
Hi we are Stoke on Trent! Eek I thought it maybe would get through an MOT!
Will have to take it to Mercx I think and get them to check it for us
many Thanks
I know this is an old thread, but perhaps for future use: I had the same problem, SRS light permanently on. For me, the fix was easy and cheap. I disconnected the three connectors under the passenger seat, sprayed some WD40 on them and reconnected them. Light went off. I'm not saying this would fix it for everybody, obviously, but worth trying before anything else, as it's both cheap and easy to do.