I used to work for TXU Energi (previously Powergen and now E-On) during the summer holidays at uni, and I think the way they can change between suppliers is ridiculous.
From memory and how it was (it might have changed), there is a 6 week transfer period and this can be agreed over the phone with no need to sign anything. If the previous owner changed to EOn 2 days before he left and you moved in, your attempt at switching to BG may have been cancelled because a switch was in place. If the old owner/tenant changed and didn't pull out in 14 days, and settled his bill, the transfer would have happened without you knowing. However, if they didn't issue 'welcome to us' letters and BG ever got in touch to say the transfer failed, you're in the dark from both sides (and the letters may have come in HIS name, so you may have returned to sender anyway).
Now then, a transfer to a new company can only happen if the last bill was settled, so unless they have messed up, the previous man should have balanced his account or the transfer would have been halted from the supplier before he changed to EOn.
What's disgusting is that the company you want to go with (BG) don't seem to be good at telling people whenever the move is halted - and considering how much they pester you if you try to leave, you'd think they'd be more keen to keep new potential customers.
Call OFGEM or Energy Watch first of all, but go to them armed with the facts: who you thought it was, who it is and the fact they've never billed you etc.
There is a chance you can nullify the money (I remember some people did this when I worked there yonks ago for major errors in setting up, but it depends on the circumstance). Alternatively, they may be able to arrange a re-payment plan for you instead of demanding the full amount.
It's awful how often this happens. And I will say that Scottish Power have been AMAZING for me since I changed to them. They answer calls quickly and (gasp) return calls and the service is great.
Good luck xx