The refill plug requires an Allen (hex) key, but there is limited clearance between diff casing and fuel tank. Fortunately I had a correct size key with short shank one end which could be engaged with plug. A ring spanner over the longer shank gave enough purchase to undo.
BTW the drain plug hex will likely be covered in underseal which will need to be removed to allow easy fit to correct socket.
If any garage refuses to change diff oil on one of the simplest cars around I would be concerned what work they could undertake
