Contacted the supplier and after a few questions to confirm it was dead, they arranged for Hitachi to drop off a replacement and collect the broken driver. This was done the day after making the call (Friday) and I now have a working impact driver, and can confirm I'm very pleased with it. It does the job I've failed to do with a manual screwdriver, a drill + screwdriver bit and a manual impact driver - remove screws holding panels into a 4 year old fence. Awesome
When the new driver was delivered, it also wouldn't work with either battery, however the batteries charged without any problem and all was okay, so it is possible the charger of the previous driver was the problem.