Sorry, I missed this earlier.
@Crisps your consultant has faulty reasoning. A situation like that is known as a fibrous pseudoarthrosis or fibrous false joint. It will never convert to a bony fusion because the 'joint' will always be subjected to micro movement caused by the pulling of the muscle attachments on the trochanter which will not be conducive to bone growth. The only remedy is to have the osteotomy opened up, the fibrous material scraped out and then fixed with some kind of internal fixation such as screws or wires. And also use of bone graft in some form.