I haven't done exactly this transfer, but I've had experience with LHR and Stansted and National Express (on the LGW-LHR route and the Victoria-STN route and vice versa) and the M25. This simply does not look good to me at all. If you're lucky it will work, but that's all I could say. How much money will you lose if you miss your flight home?

Likely bottlenecks:
1) Your flight to LHR is late. (Seems to always happen to me, although I suppose I must have had an on-time flight at some point.)
2) You are delayed at the gate. (Once this delay at LHR was 85 minutes for me, before I was allowed off the plane and on to a shuttle bus to the terminal.)
3) You are delayed at immigration (unless this is an intra-UK flight).
4) The National Express bus is delayed. Not likely to be a huge problem, as there are always more of them, but they do not run to schedule, in my experience. The bus drivers have a terrible attitude, as there seems to be labor relations problems. With how close you are cutting this, a very reliable booked car service might be a good idea. Every time, even on a late Sunday night, it's taken longer than the scheduled time.
5) When you arrive at Stansted, it will take a few minutes to get into the terminal and find your spot.