Hi Andy
I may be wrong but I thought drumlins could form anywhere along the route of the flow. You must be familiar with those lovely ice worn outcrops that you get in the bottom of many of the lakeland valleys. All that's required is for glacial till to start being deposited over the outcrop and a drumlin forms. From your photo it looks like the mound is orientated along the valley which could also support the glacial theory.

I guess another none prehistoric explanation could be a pillow mound just like those in the upper Eden valley by Pendragon Castle but I guess the smaller mounds may knock that theory on the heed.

As Mr Stonelifter says, why not drop an enquiry in to your local archaeologist and ask their opinion.