I might be in the minority here, but if SE wants to leave for Stevenage, agree a deal and let him go.
If he's unhappy - and his language indicates that, albeit very carefully - he's not going to be emotionally invested in the club performing well. And denying him now just makes it nailed on he'll leave for nothing in the summer.
I know we love the loyalty line in football but it's a façade. We've had some really positive results under SE. If he wants to go, I don't see any reason to keep him other than pig-headed stubbornness, which will almost certainly bite us further down the line (can you imagine what Evans will say about the club?).
Get your new team in, give them sometime to assess, back them building in the next two windows.