Pain control after hip replacement surgery is important to ensure patient comfort, allow mobilisation, and aid recovery. The investigators propose a simple and pragmatic study comparing two different anaesthetic techniques in the provision of pain relief after hip surgery. Patients will be randomised to receive either spinal anaesthesia containing morphine or spinal anaesthesia without morphine and an ultrasound guided fascia iliaca nerve block. Although morphine is an effective pain killer, its side effects include itch, urinary retention, nausea and potentially fatal breathing problems. If the nerve block can be shown to provide comparable pain relief to spinal morphine, then morphine could be removed from the spinal injection. This could reduce side effects and improve patient safety. The investigators wish to investigate whether ultrasound guided fascia iliaca plane block provides analgesia which is comparable to that of intrathecal opioid for primary hip arthroplasty in the first 24 hours after hip replacement surgery