So... I phoned my insurance company, they said it was a hardware fault and under warranty, so take it back to the Carphone Warehouse (from whom I originally purchased it) - took it in and they said to take it to the insurance company engineers in another branch. Having done so, I was told it was not a warranty claimable fault. I argued that it was a known issue, and was covered by a 2 year warranty. They disappeared into the office. They reappeared with my phone, told me I was right, and after messing around with something on their computer advised that I should go back on Tuesday to receive a replacement. Not an E7 or N8 but another N900. I was amazed!

I shall update with the outcome... extremely hopeful, but highly suspicious that I will get there only to be told that they couldn't source a replacement...