I contacted the CP vet this morning and received the following reply:- Although we cannot make someone use a new needle everytime, we would strongly advise that they do. As you say, everytime the needle is inserted under the skin, it becomes more blunt. It could lead to lipohypertrophy which is a thickened, enlarged or raised portion of the skin's fatty under layer where insulin injections are given. .
The needle will not be sterile, thus increasing the risk of infection and as the needle is more blunt, injecting the cat will be that much more painful. As the silicone layer wears off, it may contaminate or interfere with the insulin’s activity as white precipitates can form in the vial. A bent needle should not be used again.
The pictures they have also sent me would make anyone who injects not want to use a needle more than once.