By Susan Knox Rock legend Sir Rod Stewart surprised staff at a five-star hotel with a £10,000 tip, before advising them how to spend it. The generous gesture was made at Gleneagles Hotel in Perthshire, where he celebrated Celtic’s victory over Rangers with his wife and two sons. Sir Rod, 78, who has topped the UK charts six times, visited his local NHS hospital in Essex last year to personally pay for patients’ scans to help reduce waiting lists. The rocker and his family were actually living it up in a five-star hotel in Scotland \- enjoying their “first ever #hogmanay” and embracing their Sc…