surgery

i was injured in april 2009 after a car went into the back of me. after numerous physio sessions, cortisone injections and mris the medical reports state i need arhoscopic shoulder surgery for a torn labrum and impingement. the otherside insurance company have paid all costs to date but are refusing now to pay for the surgery that has been quoted for just over five thousand pounds (liabilty has always been admitted), they have told my solicitor they need a receipt before they pay but we cant get a receipt untill after, so now my solictors have issued proceddings and will be requesting the court for intrim payment so as i can get the surgery done my question is why are they noww refusing to comply my solicitor is very pleasent and nice but even in her own words she hasnt dealt with many rta claims and im worried that at this stage and still no surgery that my case is not going how it should any advice would be appreciated.thank you