If it's real mutton - old ewe for instance - then long and slow, yes. Shoulder needs longer and slower than leg. If you've got an Aga or similar with a slow oven, start it off in the main oven at roughly 165C (covered) for 45 mins, then leave it in the slow oven all day, or overnight. If using electric, I'd do shoulder at least 5 hours, leg at least 3, at say 140C, after that initial 45mins at 165C.