A 65-year-old male presents to you in the GP surgery complaining of fatigue that has been getting worse over the last few months. He is also complaining of strange bruises that keep appearing all over his body and he does not remember bumping into anything. His heart and lungs sound clear on auscultation however you notice that he has an enlarged spleen. A full blood count reveals that his haemoglobin levels are elevated at 198g/L. What is the most likely diagnosis?