A 63 year old man is breathless on exertion and low in mood. He takes inhalers for asthma. He drives a lorry and finds it difficult to maintain a healthy diet due to his shift work. He has pallor of the conjunctiva, and a red sore tongue. He has subjective numbness at the tips of his toes. 

Investigations:

Haemoglobin            88g/L (115-165)
MCV                        104 fL (80-96)

Blood film Hypersegmented megaloblastic cells

Which investigation is most likely to confirm the diagnosis?