Well, I went to Carphone Warehouse in Streatham and they said I couldn't upgrade irrespective of my willingness to pay, so I got myself a replacement handset. It's a bit nicer than my previous one but doesn't have any additional functionality. I had checked with the insurance department earlier and they had said that accepting a new handset would not cancel my complaint nor make me wait longer for an upgrade.
When I have finished with this ie got a Blackberry either by satisfactory resolution of my complaint or switching to a new provider, Jimmy can have this.