Was up that way yesterday, Mel. I was to the S of Bannerdale Crags and could see the faint path coming down the steep grassy hillside - didn't look too bad.
An alternative would be to head west from Bannerdale Crags summit to the head of the Glenderamackin valley, then take the path that traverses the slopes above the south side of the valley. We climbed up this path yesterday, and it is a good route, with nice views down the valley. There is one short stretch where it is a bit boggy, but it soon improves again, and as you descend it meets up with the path from Scales Tarn/Sharp Edge, which brings you easily onto the ridge leading to Souther Fell, thereby avoiding the reascent from the Glenderamackin Bridge. It is a long way around, though.