A 26 year old man presents with a swollen, painful left calf which developed over 24 hours after a long haul flight. He has no past medical history and is not taking any regular medication. Examination shows a warm, tender calf with pitting oedema to the knee. His Wells score suggests a likely deep vein thrombosis. Immediate ultrasound is not available out of hours. What is the most appropriate next step in management?