A 50-year-old woman presents with a 3-month history of fatigue, generalised aches, and low mood. Blood tests reveal: Haemoglobin 105 g/L (Ref: 115-165), MCV 108 fL (Ref: 80-100), Serum B12 <150 ng/L (Ref: 197-771), Serum Folate normal. What is the most appropriate initial treatment?