I am surprised that HSBC has failed to involve the police where the transaction originated. If your branch manager and possibly their ombudsman/ customer advocate if any are failing to respond constructively, I would start looking for a friendly local reporter -- phone ther news desk of your local papers/TV outlets. You may be pleasantly surprised once said reporter starts phoning head office.
Or find a junkyard dog tort lawyer with a flair for publicity, but he will likely take 30%.