Well even in Leicester, we have a lot of people who have and do use Stansted. A6, A14 and M11 and you're there.
The A14 has a bad reputation due to a high level of lorries using it going to and from the port at Felixstowe. I've used it many times and only had traffic problems once, and that was back in 2005. Unfortuneately, the same can't be said for the M1, especially round the Luton J9 and 10 area. Although recently improved vastly, congestion still occurs.
If a number of people in Leicester go to Stansted when we have East Midlands (FR's 2nd or 3rd largest UK base), Birmingham and Luton closer, I'm sure those from Northampton will also use or at least consider Stansted.