This happens more often than we'd care to think about. Farmer's with large flocks quite simply find such cases a nuisance, they shove them in a shed and 'forget' about them. There is no excuse for neglect which this obviously is, however, so I'd definitely report them to Animal Health or RSPCA or both, even if its just to ensure all their other animals are being kept properly. These people don't deserve to be keeping animals. Furthermore, if this farm is of the size to be claiming any sort of subsidies, then definitely Animal Health as the taxpayer is indirectly supporting them.
I really hope the little lamb makes it, sounds like you're doing all you can - keep us posted won't you.