The investigators hypothesise that by increasing the success rate of the fascia iliaca block with ultrasound, it will be possible to achieve superior analgesia post-operatively. Our aim is to assess whether the ultrasound guided fascia iliaca plane block can be used as an alternative to intrathecal morphine in the provision of post-operative analgesia for primary hip arthroplasty. If this is the case, intrathecal opioid could be removed from the spinal anaesthetic. This could in theory have significant safety benefits whilst also reducing side effects. Ultrasound guided fascia iliaca block has not yet been evaluated clinically as a method of providing post-operative analgesia following primary hip arthroplasty. The investigators believe that further investigation of this technique will provide a valuable contribution to existing knowledge and will change current practice.