Have you undergone electrocochleaography yet? Forgive me if I'm stating the obvious. My GP says it's a very useful test to have for diagnosing inner ear issues. It's something I underwent in investigating my suspected inner ear issue.
It might be useful if you were to list all the test procedures you've been through, Stilton - that way we won't have to tread over old ground.
Edit - have you thought of trying a course of Betahistine tablets? Although the drug is associated as a treatment for Meniere's, it's designed to improve blood flow in the inner ear. I have heard it suggested that it *could* have uses treating people exposed to loud trauma, if given immediately afterwards. The side effects of Betahistine are supposed to be very mild, and Meniere's sufferers take it long term. Could be worth investigating?...